Revision: 1117
http://www.exim.org/viewvc/pcre2?view=rev&revision=1117
Author: ph10
Date: 2019-06-21 17:10:17 +0100 (Fri, 21 Jun 2019)
Log Message:
-----------
Documentation update.
Modified Paths:
--------------
code/trunk/doc/html/pcre2partial.html
code/trunk/doc/html/pcre2pattern.html
code/trunk/doc/pcre2.txt
code/trunk/doc/pcre2partial.3
code/trunk/doc/pcre2pattern.3
Modified: code/trunk/doc/html/pcre2partial.html
===================================================================
--- code/trunk/doc/html/pcre2partial.html 2019-06-20 17:19:13 UTC (rev 1116)
+++ code/trunk/doc/html/pcre2partial.html 2019-06-21 16:10:17 UTC (rev 1117)
@@ -355,7 +355,14 @@
</PRE>
</P>
<P>
-3. Because a partial match must always contain at least one character, what
+3. The maximum lookbehind count is also important when the result of a partial
+match attempt is "no match". In this case, the maximum lookbehind characters
+from the end of the current segment must be retained at the start of the next
+segment, in case the lookbehind is at the start of the pattern. Matching the
+next segment must then start at the appropriate offset.
+</P>
+<P>
+4. Because a partial match must always contain at least one character, what
might be considered a partial match of an empty string actually gives a "no
match" result. For example:
<pre>
@@ -369,7 +376,7 @@
when the pattern contains lookbehinds.
</P>
<P>
-4. Matching a subject string that is split into multiple segments may not
+5. Matching a subject string that is split into multiple segments may not
always produce exactly the same result as matching over one single long string,
especially when PCRE2_PARTIAL_SOFT is used. The section "Partial Matching and
Word Boundaries" above describes an issue that arises if the pattern ends with
@@ -411,7 +418,7 @@
data> gsb\=ph,dfa,dfa_restart
Partial match: gsb
</pre>
-5. Patterns that contain alternatives at the top level which do not all start
+6. Patterns that contain alternatives at the top level which do not all start
with the same pattern item may not work as expected when PCRE2_DFA_RESTART is
used. For example, consider this pattern:
<pre>
@@ -456,9 +463,9 @@
</P>
<br><a name="SEC10" href="#TOC1">REVISION</a><br>
<P>
-Last updated: 22 December 2014
+Last updated: 21 June 2019
<br>
-Copyright © 1997-2014 University of Cambridge.
+Copyright © 1997-2019 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE2 index page</a>.
Modified: code/trunk/doc/html/pcre2pattern.html
===================================================================
--- code/trunk/doc/html/pcre2pattern.html 2019-06-20 17:19:13 UTC (rev 1116)
+++ code/trunk/doc/html/pcre2pattern.html 2019-06-21 16:10:17 UTC (rev 1117)
@@ -2014,8 +2014,10 @@
</pre>
Earlier versions of Perl and PCRE1 used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
-patterns are now accepted, but if any repetition of the group does in fact
-match no characters, the loop is forcibly broken.
+patterns are now accepted, but whenever an iteration of such a group matches no
+characters, matching moves on to the next item in the pattern instead of
+repeatedly matching an empty string. This does not prevent backtracking into
+any of the iterations if a subsequent item fails to match.
</P>
<P>
By default, quantifiers are "greedy", that is, they match as much as possible
@@ -2371,6 +2373,10 @@
which branch of the condition is followed.
</P>
<P>
+Lookaround assertions are atomic. If an assertion is true, but there is a
+subsequent matching failure, there is no backtracking into the assertion.
+</P>
+<P>
Assertion groups are not capture groups. If an assertion contains capture
groups within it, these are counted for the purposes of numbering the capture
groups in the whole pattern. Within each branch of an assertion, locally
@@ -3519,9 +3525,9 @@
instead of skipping on to "c".
</P>
<P>
-If (*SKIP) is used inside a lookbehind to specify a new starting point that is
-not later than the starting point of the current match, it is ignored, and the
-normal "bumpalong" occurs.
+If (*SKIP) is used inside a lookbehind to specify a new starting position that
+is not later than the starting point of the current match, the position
+specified by (*SKIP) is ignored, and instead the normal "bumpalong" occurs.
<pre>
(*SKIP:NAME)
</pre>
@@ -3748,7 +3754,7 @@
</P>
<br><a name="SEC31" href="#TOC1">REVISION</a><br>
<P>
-Last updated: 20 June 2019
+Last updated: 21 June 2019
<br>
Copyright © 1997-2019 University of Cambridge.
<br>
Modified: code/trunk/doc/pcre2.txt
===================================================================
--- code/trunk/doc/pcre2.txt 2019-06-20 17:19:13 UTC (rev 1116)
+++ code/trunk/doc/pcre2.txt 2019-06-21 16:10:17 UTC (rev 1117)
@@ -5970,8 +5970,15 @@
Partial match: 123ab
<<<
- 3. Because a partial match must always contain at least one character,
- what might be considered a partial match of an empty string actually
+ 3. The maximum lookbehind count is also important when the result of a
+ partial match attempt is "no match". In this case, the maximum lookbe-
+ hind characters from the end of the current segment must be retained at
+ the start of the next segment, in case the lookbehind is at the start
+ of the pattern. Matching the next segment must then start at the appro-
+ priate offset.
+
+ 4. Because a partial match must always contain at least one character,
+ what might be considered a partial match of an empty string actually
gives a "no match" result. For example:
re> /c(?<=abc)x/
@@ -5979,19 +5986,19 @@
No match
If the next segment begins "cx", a match should be found, but this will
- only happen if characters from the previous segment are retained. For
- this reason, a "no match" result should be interpreted as "partial
+ only happen if characters from the previous segment are retained. For
+ this reason, a "no match" result should be interpreted as "partial
match of an empty string" when the pattern contains lookbehinds.
- 4. Matching a subject string that is split into multiple segments may
- not always produce exactly the same result as matching over one single
- long string, especially when PCRE2_PARTIAL_SOFT is used. The section
- "Partial Matching and Word Boundaries" above describes an issue that
- arises if the pattern ends with \b or \B. Another kind of difference
- may occur when there are multiple matching possibilities, because (for
+ 5. Matching a subject string that is split into multiple segments may
+ not always produce exactly the same result as matching over one single
+ long string, especially when PCRE2_PARTIAL_SOFT is used. The section
+ "Partial Matching and Word Boundaries" above describes an issue that
+ arises if the pattern ends with \b or \B. Another kind of difference
+ may occur when there are multiple matching possibilities, because (for
PCRE2_PARTIAL_SOFT) a partial match result is given only when there are
no completed matches. This means that as soon as the shortest match has
- been found, continuation to a new subject segment is no longer possi-
+ been found, continuation to a new subject segment is no longer possi-
ble. Consider this pcre2test example:
re> /dog(sbody)?/
@@ -6005,18 +6012,18 @@
0: dogsbody
1: dog
- The first data line passes the string "dogsb" to a standard matching
+ The first data line passes the string "dogsb" to a standard matching
function, setting the PCRE2_PARTIAL_SOFT option. Although the string is
- a partial match for "dogsbody", the result is not PCRE2_ERROR_PARTIAL,
- because the shorter string "dog" is a complete match. Similarly, when
- the subject is presented to a DFA matching function in several parts
- ("do" and "gsb" being the first two) the match stops when "dog" has
- been found, and it is not possible to continue. On the other hand, if
- "dogsbody" is presented as a single string, a DFA matching function
+ a partial match for "dogsbody", the result is not PCRE2_ERROR_PARTIAL,
+ because the shorter string "dog" is a complete match. Similarly, when
+ the subject is presented to a DFA matching function in several parts
+ ("do" and "gsb" being the first two) the match stops when "dog" has
+ been found, and it is not possible to continue. On the other hand, if
+ "dogsbody" is presented as a single string, a DFA matching function
finds both matches.
- Because of these problems, it is best to use PCRE2_PARTIAL_HARD when
- matching multi-segment data. The example above then behaves differ-
+ Because of these problems, it is best to use PCRE2_PARTIAL_HARD when
+ matching multi-segment data. The example above then behaves differ-
ently:
re> /dog(sbody)?/
@@ -6027,26 +6034,26 @@
data> gsb\=ph,dfa,dfa_restart
Partial match: gsb
- 5. Patterns that contain alternatives at the top level which do not all
- start with the same pattern item may not work as expected when
+ 6. Patterns that contain alternatives at the top level which do not all
+ start with the same pattern item may not work as expected when
PCRE2_DFA_RESTART is used. For example, consider this pattern:
1234|3789
- If the first part of the subject is "ABC123", a partial match of the
- first alternative is found at offset 3. There is no partial match for
+ If the first part of the subject is "ABC123", a partial match of the
+ first alternative is found at offset 3. There is no partial match for
the second alternative, because such a match does not start at the same
- point in the subject string. Attempting to continue with the string
- "7890" does not yield a match because only those alternatives that
- match at one point in the subject are remembered. The problem arises
- because the start of the second alternative matches within the first
- alternative. There is no problem with anchored patterns or patterns
+ point in the subject string. Attempting to continue with the string
+ "7890" does not yield a match because only those alternatives that
+ match at one point in the subject are remembered. The problem arises
+ because the start of the second alternative matches within the first
+ alternative. There is no problem with anchored patterns or patterns
such as:
1234|ABCD
- where no string can be a partial match for both alternatives. This is
- not a problem if a standard matching function is used, because the
+ where no string can be a partial match for both alternatives. This is
+ not a problem if a standard matching function is used, because the
entire match has to be rerun each time:
re> /1234|3789/
@@ -6055,11 +6062,11 @@
data> 1237890
0: 3789
- Of course, instead of using PCRE2_DFA_RESTART, the same technique of
- re-running the entire match can also be used with the DFA matching
+ Of course, instead of using PCRE2_DFA_RESTART, the same technique of
+ re-running the entire match can also be used with the DFA matching
function. Another possibility is to work with two buffers. If a partial
- match at offset n in the first buffer is followed by "no match" when
- PCRE2_DFA_RESTART is used on the second buffer, you can then try a new
+ match at offset n in the first buffer is followed by "no match" when
+ PCRE2_DFA_RESTART is used on the second buffer, you can then try a new
match starting at offset n+1 in the first buffer.
@@ -6072,8 +6079,8 @@
REVISION
- Last updated: 22 December 2014
- Copyright (c) 1997-2014 University of Cambridge.
+ Last updated: 21 June 2019
+ Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
@@ -7801,15 +7808,18 @@
Earlier versions of Perl and PCRE1 used to give an error at compile
time for such patterns. However, because there are cases where this can
- be useful, such patterns are now accepted, but if any repetition of the
- group does in fact match no characters, the loop is forcibly broken.
+ be useful, such patterns are now accepted, but whenever an iteration of
+ such a group matches no characters, matching moves on to the next item
+ in the pattern instead of repeatedly matching an empty string. This
+ does not prevent backtracking into any of the iterations if a subse-
+ quent item fails to match.
- By default, quantifiers are "greedy", that is, they match as much as
+ By default, quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing
- the rest of the pattern to fail. The classic example of where this
- gives problems is in trying to match comments in C programs. These
- appear between /* and */ and within the comment, individual * and /
- characters may appear. An attempt to match C comments by applying the
+ the rest of the pattern to fail. The classic example of where this
+ gives problems is in trying to match comments in C programs. These
+ appear between /* and */ and within the comment, individual * and /
+ characters may appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
@@ -7818,17 +7828,17 @@
/* first comment */ not comment /* second comment */
- fails, because it matches the entire string owing to the greediness of
- the .* item. However, if a quantifier is followed by a question mark,
+ fails, because it matches the entire string owing to the greediness of
+ the .* item. However, if a quantifier is followed by a question mark,
it ceases to be greedy, and instead matches the minimum number of times
possible, so the pattern
/\*.*?\*/
- does the right thing with the C comments. The meaning of the various
- quantifiers is not otherwise changed, just the preferred number of
- matches. Do not confuse this use of question mark with its use as a
- quantifier in its own right. Because it has two uses, it can sometimes
+ does the right thing with the C comments. The meaning of the various
+ quantifiers is not otherwise changed, just the preferred number of
+ matches. Do not confuse this use of question mark with its use as a
+ quantifier in its own right. Because it has two uses, it can sometimes
appear doubled, as in
\d??\d
@@ -7837,55 +7847,55 @@
only way the rest of the pattern matches.
If the PCRE2_UNGREEDY option is set (an option that is not available in
- Perl), the quantifiers are not greedy by default, but individual ones
- can be made greedy by following them with a question mark. In other
+ Perl), the quantifiers are not greedy by default, but individual ones
+ can be made greedy by following them with a question mark. In other
words, it inverts the default behaviour.
- When a parenthesized group is quantified with a minimum repeat count
- that is greater than 1 or with a limited maximum, more memory is
- required for the compiled pattern, in proportion to the size of the
+ When a parenthesized group is quantified with a minimum repeat count
+ that is greater than 1 or with a limited maximum, more memory is
+ required for the compiled pattern, in proportion to the size of the
minimum or maximum.
- If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
- (equivalent to Perl's /s) is set, thus allowing the dot to match new-
- lines, the pattern is implicitly anchored, because whatever follows
- will be tried against every character position in the subject string,
- so there is no point in retrying the overall match at any position
+ If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option
+ (equivalent to Perl's /s) is set, thus allowing the dot to match new-
+ lines, the pattern is implicitly anchored, because whatever follows
+ will be tried against every character position in the subject string,
+ so there is no point in retrying the overall match at any position
after the first. PCRE2 normally treats such a pattern as though it were
preceded by \A.
- In cases where it is known that the subject string contains no new-
- lines, it is worth setting PCRE2_DOTALL in order to obtain this opti-
+ In cases where it is known that the subject string contains no new-
+ lines, it is worth setting PCRE2_DOTALL in order to obtain this opti-
mization, or alternatively, using ^ to indicate anchoring explicitly.
- However, there are some cases where the optimization cannot be used.
- When .* is inside capturing parentheses that are the subject of a
- backreference elsewhere in the pattern, a match at the start may fail
+ However, there are some cases where the optimization cannot be used.
+ When .* is inside capturing parentheses that are the subject of a
+ backreference elsewhere in the pattern, a match at the start may fail
where a later one succeeds. Consider, for example:
(.*)abc\1
- If the subject is "xyz123abc123" the match point is the fourth charac-
+ If the subject is "xyz123abc123" the match point is the fourth charac-
ter. For this reason, such a pattern is not implicitly anchored.
- Another case where implicit anchoring is not applied is when the lead-
- ing .* is inside an atomic group. Once again, a match at the start may
+ Another case where implicit anchoring is not applied is when the lead-
+ ing .* is inside an atomic group. Once again, a match at the start may
fail where a later one succeeds. Consider this pattern:
(?>.*?a)b
- It matches "ab" in the subject "aab". The use of the backtracking con-
- trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and
+ It matches "ab" in the subject "aab". The use of the backtracking con-
+ trol verbs (*PRUNE) and (*SKIP) also disable this optimization, and
there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
- When a capture group is repeated, the value captured is the substring
+ When a capture group is repeated, the value captured is the substring
that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee" the value of the captured substring
- is "tweedledee". However, if there are nested capture groups, the cor-
- responding captured values may have been set in previous iterations.
+ is "tweedledee". However, if there are nested capture groups, the cor-
+ responding captured values may have been set in previous iterations.
For example, after
(a|(b))+
@@ -7895,33 +7905,33 @@
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
- With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
- repetition, failure of what follows normally causes the repeated item
- to be re-evaluated to see if a different number of repeats allows the
- rest of the pattern to match. Sometimes it is useful to prevent this,
- either to change the nature of the match, or to cause it fail earlier
- than it otherwise might, when the author of the pattern knows there is
+ With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
+ repetition, failure of what follows normally causes the repeated item
+ to be re-evaluated to see if a different number of repeats allows the
+ rest of the pattern to match. Sometimes it is useful to prevent this,
+ either to change the nature of the match, or to cause it fail earlier
+ than it otherwise might, when the author of the pattern knows there is
no point in carrying on.
- Consider, for example, the pattern \d+foo when applied to the subject
+ Consider, for example, the pattern \d+foo when applied to the subject
line
123456bar
After matching all 6 digits and then failing to match "foo", the normal
- action of the matcher is to try again with only 5 digits matching the
- \d+ item, and then with 4, and so on, before ultimately failing.
- "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
+ action of the matcher is to try again with only 5 digits matching the
+ \d+ item, and then with 4, and so on, before ultimately failing.
+ "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
the means for specifying that once a group has matched, it is not to be
re-evaluated in this way.
- If we use atomic grouping for the previous example, the matcher gives
- up immediately on failing to match "foo" the first time. The notation
+ If we use atomic grouping for the previous example, the matcher gives
+ up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in this example:
(?>\d+)foo
- Perl 5.28 introduced an experimental alphabetic form starting with (*
+ Perl 5.28 introduced an experimental alphabetic form starting with (*
which may be easier to remember:
(*atomic:\d+)foo
@@ -7928,24 +7938,24 @@
This kind of parenthesized group "locks up" the part of the pattern it
contains once it has matched, and a failure further into the pattern is
- prevented from backtracking into it. Backtracking past it to previous
+ prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a group of this type matches exactly
- the string of characters that an identical standalone pattern would
+ the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
- Atomic groups are not capture groups. Simple cases such as the above
- example can be thought of as a maximizing repeat that must swallow
- everything it can. So, while both \d+ and \d+? are prepared to adjust
- the number of digits they match in order to make the rest of the pat-
+ Atomic groups are not capture groups. Simple cases such as the above
+ example can be thought of as a maximizing repeat that must swallow
+ everything it can. So, while both \d+ and \d+? are prepared to adjust
+ the number of digits they match in order to make the rest of the pat-
tern match, (?>\d+) can only match an entire sequence of digits.
- Atomic groups in general can of course contain arbitrarily complicated
+ Atomic groups in general can of course contain arbitrarily complicated
expressions, and can be nested. However, when the contents of an atomic
- group is just a single repeated item, as in the example above, a sim-
- pler notation, called a "possessive quantifier" can be used. This con-
- sists of an additional + character following a quantifier. Using this
+ group is just a single repeated item, as in the example above, a sim-
+ pler notation, called a "possessive quantifier" can be used. This con-
+ sists of an additional + character following a quantifier. Using this
notation, the previous example can be rewritten as
\d++foo
@@ -7955,46 +7965,46 @@
(abc|xyz){2,3}+
- Possessive quantifiers are always greedy; the setting of the
- PCRE2_UNGREEDY option is ignored. They are a convenient notation for
- the simpler forms of atomic group. However, there is no difference in
+ Possessive quantifiers are always greedy; the setting of the
+ PCRE2_UNGREEDY option is ignored. They are a convenient notation for
+ the simpler forms of atomic group. However, there is no difference in
the meaning of a possessive quantifier and the equivalent atomic group,
- though there may be a performance difference; possessive quantifiers
+ though there may be a performance difference; possessive quantifiers
should be slightly faster.
- The possessive quantifier syntax is an extension to the Perl 5.8 syn-
- tax. Jeffrey Friedl originated the idea (and the name) in the first
+ The possessive quantifier syntax is an extension to the Perl 5.8 syn-
+ tax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
- built Sun's Java package, and PCRE1 copied it from there. It found its
+ built Sun's Java package, and PCRE1 copied it from there. It found its
way into Perl at release 5.10.
- PCRE2 has an optimization that automatically "possessifies" certain
- simple pattern constructs. For example, the sequence A+B is treated as
- A++B because there is no point in backtracking into a sequence of A's
+ PCRE2 has an optimization that automatically "possessifies" certain
+ simple pattern constructs. For example, the sequence A+B is treated as
+ A++B because there is no point in backtracking into a sequence of A's
when B must follow. This feature can be disabled by the PCRE2_NO_AUTO-
POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).
- When a pattern contains an unlimited repeat inside a group that can
- itself be repeated an unlimited number of times, the use of an atomic
- group is the only way to avoid some failing matches taking a very long
+ When a pattern contains an unlimited repeat inside a group that can
+ itself be repeated an unlimited number of times, the use of an atomic
+ group is the only way to avoid some failing matches taking a very long
time indeed. The pattern
(\D+|<\d+>)*[!?]
- matches an unlimited number of substrings that either consist of non-
- digits, or digits enclosed in <>, followed by either ! or ?. When it
+ matches an unlimited number of substrings that either consist of non-
+ digits, or digits enclosed in <>, followed by either ! or ?. When it
matches, it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
- it takes a long time before reporting failure. This is because the
- string can be divided between the internal \D+ repeat and the external
- * repeat in a large number of ways, and all have to be tried. (The
- example uses [!?] rather than a single character at the end, because
- both PCRE2 and Perl have an optimization that allows for fast failure
- when a single character is used. They remember the last single charac-
- ter that is required for a match, and fail early if it is not present
- in the string.) If the pattern is changed so that it uses an atomic
+ it takes a long time before reporting failure. This is because the
+ string can be divided between the internal \D+ repeat and the external
+ * repeat in a large number of ways, and all have to be tried. (The
+ example uses [!?] rather than a single character at the end, because
+ both PCRE2 and Perl have an optimization that allows for fast failure
+ when a single character is used. They remember the last single charac-
+ ter that is required for a match, and fail early if it is not present
+ in the string.) If the pattern is changed so that it uses an atomic
group, like this:
((?>\D+)|<\d+>)*[!?]
@@ -8005,28 +8015,28 @@
BACKREFERENCES
Outside a character class, a backslash followed by a digit greater than
- 0 (and possibly further digits) is a backreference to a capture group
+ 0 (and possibly further digits) is a backreference to a capture group
earlier (that is, to its left) in the pattern, provided there have been
that many previous capture groups.
- However, if the decimal number following the backslash is less than 8,
- it is always taken as a backreference, and causes an error only if
- there are not that many capture groups in the entire pattern. In other
+ However, if the decimal number following the backslash is less than 8,
+ it is always taken as a backreference, and causes an error only if
+ there are not that many capture groups in the entire pattern. In other
words, the group that is referenced need not be to the left of the ref-
- erence for numbers less than 8. A "forward backreference" of this type
+ erence for numbers less than 8. A "forward backreference" of this type
can make sense when a repetition is involved and the group to the right
has participated in an earlier iteration.
- It is not possible to have a numerical "forward backreference" to a
- group whose number is 8 or more using this syntax because a sequence
- such as \50 is interpreted as a character defined in octal. See the
+ It is not possible to have a numerical "forward backreference" to a
+ group whose number is 8 or more using this syntax because a sequence
+ such as \50 is interpreted as a character defined in octal. See the
subsection entitled "Non-printing characters" above for further details
- of the handling of digits following a backslash. Other forms of back-
- referencing do not suffer from this restriction. In particular, there
+ of the handling of digits following a backslash. Other forms of back-
+ referencing do not suffer from this restriction. In particular, there
is no problem when named capture groups are used (see below).
- Another way of avoiding the ambiguity inherent in the use of digits
- following a backslash is to use the \g escape sequence. This escape
+ Another way of avoiding the ambiguity inherent in the use of digits
+ following a backslash is to use the \g escape sequence. This escape
must be followed by a signed or unsigned number, optionally enclosed in
braces. These examples are all identical:
@@ -8034,9 +8044,9 @@
(ring), \g1
(ring), \g{1}
- An unsigned number specifies an absolute reference without the ambigu-
+ An unsigned number specifies an absolute reference without the ambigu-
ity that is present in the older syntax. It is also useful when literal
- digits follow the reference. A signed number is a relative reference.
+ digits follow the reference. A signed number is a relative reference.
Consider this example:
(abc(def)ghi)\g{-1}
@@ -8044,36 +8054,36 @@
The sequence \g{-1} is a reference to the most recently started capture
group before \g, that is, is it equivalent to \2 in this example. Simi-
larly, \g{-2} would be equivalent to \1. The use of relative references
- can be helpful in long patterns, and also in patterns that are created
- by joining together fragments that contain references within them-
+ can be helpful in long patterns, and also in patterns that are created
+ by joining together fragments that contain references within them-
selves.
The sequence \g{+1} is a reference to the next capture group. This kind
- of forward reference can be useful in patterns that repeat. Perl does
+ of forward reference can be useful in patterns that repeat. Perl does
not support the use of + in this way.
- A backreference matches whatever actually most recently matched the
- capture group in the current subject string, rather than anything at
+ A backreference matches whatever actually most recently matched the
+ capture group in the current subject string, rather than anything at
all that matches the group (see "Groups as subroutines" below for a way
of doing that). So the pattern
(sens|respons)e and \1ibility
- matches "sense and sensibility" and "response and responsibility", but
- not "sense and responsibility". If caseful matching is in force at the
- time of the backreference, the case of letters is relevant. For exam-
+ matches "sense and sensibility" and "response and responsibility", but
+ not "sense and responsibility". If caseful matching is in force at the
+ time of the backreference, the case of letters is relevant. For exam-
ple,
((?i)rah)\s+\1
- matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
+ matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
original capture group is matched caselessly.
- There are several different ways of writing backreferences to named
- capture groups. The .NET syntax \k{name} and the Perl syntax \k<name>
- or \k'name' are supported, as is the Python syntax (?P=name). Perl
- 5.10's unified backreference syntax, in which \g can be used for both
- numeric and named references, is also supported. We could rewrite the
+ There are several different ways of writing backreferences to named
+ capture groups. The .NET syntax \k{name} and the Perl syntax \k<name>
+ or \k'name' are supported, as is the Python syntax (?P=name). Perl
+ 5.10's unified backreference syntax, in which \g can be used for both
+ numeric and named references, is also supported. We could rewrite the
above example in any of the following ways:
(?<p1>(?i)rah)\s+\k<p1>
@@ -8081,31 +8091,31 @@
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
- A capture group that is referenced by name may appear in the pattern
+ A capture group that is referenced by name may appear in the pattern
before or after the reference.
- There may be more than one backreference to the same group. If a group
- has not actually been used in a particular match, backreferences to it
+ There may be more than one backreference to the same group. If a group
+ has not actually been used in a particular match, backreferences to it
always fail by default. For example, the pattern
(a|(bc))\2
- always fails if it starts to match "a" rather than "bc". However, if
+ always fails if it starts to match "a" rather than "bc". However, if
the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
erence to an unset value matches an empty string.
- Because there may be many capture groups in a pattern, all digits fol-
- lowing a backslash are taken as part of a potential backreference num-
- ber. If the pattern continues with a digit character, some delimiter
- must be used to terminate the backreference. If the PCRE2_EXTENDED or
- PCRE2_EXTENDED_MORE option is set, this can be white space. Otherwise,
+ Because there may be many capture groups in a pattern, all digits fol-
+ lowing a backslash are taken as part of a potential backreference num-
+ ber. If the pattern continues with a digit character, some delimiter
+ must be used to terminate the backreference. If the PCRE2_EXTENDED or
+ PCRE2_EXTENDED_MORE option is set, this can be white space. Otherwise,
the \g{} syntax or an empty comment (see "Comments" below) can be used.
Recursive backreferences
- A backreference that occurs inside the group to which it refers fails
- when the group is first used, so, for example, (a\1) never matches.
- However, such references can be useful inside repeated groups. For
+ A backreference that occurs inside the group to which it refers fails
+ when the group is first used, so, for example, (a\1) never matches.
+ However, such references can be useful inside repeated groups. For
example, the pattern
(a|b\1)+
@@ -8112,86 +8122,90 @@
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the group, the backreference matches the character string cor-
- responding to the previous iteration. In order for this to work, the
- pattern must be such that the first iteration does not need to match
- the backreference. This can be done using alternation, as in the exam-
+ responding to the previous iteration. In order for this to work, the
+ pattern must be such that the first iteration does not need to match
+ the backreference. This can be done using alternation, as in the exam-
ple above, or by a quantifier with a minimum of zero.
- Backreferences of this type cause the group that they reference to be
- treated as an atomic group. Once the whole group has been matched, a
- subsequent matching failure cannot cause backtracking into the middle
+ Backreferences of this type cause the group that they reference to be
+ treated as an atomic group. Once the whole group has been matched, a
+ subsequent matching failure cannot cause backtracking into the middle
of the group.
ASSERTIONS
- An assertion is a test on the characters following or preceding the
+ An assertion is a test on the characters following or preceding the
current matching point that does not consume any characters. The simple
- assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
+ assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
above.
- More complicated assertions are coded as parenthesized groups. There
- are two kinds: those that look ahead of the current position in the
- subject string, and those that look behind it, and in each case an
- assertion may be positive (must match for the assertion to be true) or
- negative (must not match for the assertion to be true). An assertion
+ More complicated assertions are coded as parenthesized groups. There
+ are two kinds: those that look ahead of the current position in the
+ subject string, and those that look behind it, and in each case an
+ assertion may be positive (must match for the assertion to be true) or
+ negative (must not match for the assertion to be true). An assertion
group is matched in the normal way, and if it is true, matching contin-
- ues after it, but with the matching position in the subject string is
+ ues after it, but with the matching position in the subject string is
was it was before the assertion was processed.
- A lookaround assertion may also appear as the condition in a condi-
- tional group (see below). In this case, the result of matching the
+ A lookaround assertion may also appear as the condition in a condi-
+ tional group (see below). In this case, the result of matching the
assertion determines which branch of the condition is followed.
- Assertion groups are not capture groups. If an assertion contains cap-
- ture groups within it, these are counted for the purposes of numbering
- the capture groups in the whole pattern. Within each branch of an
- assertion, locally captured substrings may be referenced in the usual
- way. For example, a sequence such as (.)\g{-1} can be used to check
+ Lookaround assertions are atomic. If an assertion is true, but there is
+ a subsequent matching failure, there is no backtracking into the asser-
+ tion.
+
+ Assertion groups are not capture groups. If an assertion contains cap-
+ ture groups within it, these are counted for the purposes of numbering
+ the capture groups in the whole pattern. Within each branch of an
+ assertion, locally captured substrings may be referenced in the usual
+ way. For example, a sequence such as (.)\g{-1} can be used to check
that two adjacent characters are the same.
- When a branch within an assertion fails to match, any substrings that
- were captured are discarded (as happens with any pattern branch that
- fails to match). A negative assertion is true only when all its
+ When a branch within an assertion fails to match, any substrings that
+ were captured are discarded (as happens with any pattern branch that
+ fails to match). A negative assertion is true only when all its
branches fail to match; this means that no captured substrings are ever
- retained after a successful negative assertion. When an assertion con-
+ retained after a successful negative assertion. When an assertion con-
tains a matching branch, what happens depends on the type of assertion.
- For a positive assertion, internally captured substrings in the suc-
- cessful branch are retained, and matching continues with the next pat-
- tern item after the assertion. For a negative assertion, a matching
- branch means that the assertion is not true. If such an assertion is
- being used as a condition in a conditional group (see below), captured
- substrings are retained, because matching continues with the "no"
+ For a positive assertion, internally captured substrings in the suc-
+ cessful branch are retained, and matching continues with the next pat-
+ tern item after the assertion. For a negative assertion, a matching
+ branch means that the assertion is not true. If such an assertion is
+ being used as a condition in a conditional group (see below), captured
+ substrings are retained, because matching continues with the "no"
branch of the condition. For other failing negative assertions, control
passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
- For compatibility with Perl, most assertion groups may be repeated;
- though it makes no sense to assert the same thing several times, the
+ For compatibility with Perl, most assertion groups may be repeated;
+ though it makes no sense to assert the same thing several times, the
side effect of capturing may occasionally be useful. However, an asser-
- tion that forms the condition for a conditional group may not be quan-
+ tion that forms the condition for a conditional group may not be quan-
tified. In practice, for other assertions, there only three cases:
- (1) If the quantifier is {0}, the assertion is never obeyed during
- matching. However, it may contain internal capture groups that are
+ (1) If the quantifier is {0}, the assertion is never obeyed during
+ matching. However, it may contain internal capture groups that are
called from elsewhere via the subroutine mechanism.
- (2) If quantifier is {0,n} where n is greater than zero, it is treated
- as if it were {0,1}. At run time, the rest of the pattern match is
+ (2) If quantifier is {0,n} where n is greater than zero, it is treated
+ as if it were {0,1}. At run time, the rest of the pattern match is
tried with and without the assertion, the order depending on the greed-
iness of the quantifier.
- (3) If the minimum repetition is greater than zero, the quantifier is
- ignored. The assertion is obeyed just once when encountered during
+ (3) If the minimum repetition is greater than zero, the quantifier is
+ ignored. The assertion is obeyed just once when encountered during
matching.
Alphabetic assertion names
- Traditionally, symbolic sequences such as (?= and (?<= have been used
- to specify lookaround assertions. Perl 5.28 introduced some experimen-
+ Traditionally, symbolic sequences such as (?= and (?<= have been used
+ to specify lookaround assertions. Perl 5.28 introduced some experimen-
tal alphabetic alternatives which might be easier to remember. They all
- start with (* instead of (? and must be written using lower case let-
+ start with (* instead of (? and must be written using lower case let-
ters. PCRE2 supports the following synonyms:
(*positive_lookahead: or (*pla: is the same as (?=
@@ -8199,8 +8213,8 @@
(*positive_lookbehind: or (*plb: is the same as (?<=
(*negative_lookbehind: or (*nlb: is the same as (?<!
- For example, (*pla:foo) is the same assertion as (?=foo). In the fol-
- lowing sections, the various assertions are described using the origi-
+ For example, (*pla:foo) is the same assertion as (?=foo). In the fol-
+ lowing sections, the various assertions are described using the origi-
nal symbolic forms.
Lookahead assertions
@@ -8210,38 +8224,38 @@
\w+(?=;)
- matches a word followed by a semicolon, but does not include the semi-
+ matches a word followed by a semicolon, but does not include the semi-
colon in the match, and
foo(?!bar)
- matches any occurrence of "foo" that is not followed by "bar". Note
+ matches any occurrence of "foo" that is not followed by "bar". Note
that the apparently similar pattern
(?!foo)bar
- does not find an occurrence of "bar" that is preceded by something
- other than "foo"; it finds any occurrence of "bar" whatsoever, because
+ does not find an occurrence of "bar" that is preceded by something
+ other than "foo"; it finds any occurrence of "bar" whatsoever, because
the assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
- most convenient way to do it is with (?!) because an empty string
- always matches, so an assertion that requires there not to be an empty
+ most convenient way to do it is with (?!) because an empty string
+ always matches, so an assertion that requires there not to be an empty
string must always fail. The backtracking control verb (*FAIL) or (*F)
is a synonym for (?!).
Lookbehind assertions
- Lookbehind assertions start with (?<= for positive assertions and (?<!
+ Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example,
(?<!foo)bar
- does find an occurrence of "bar" that is not preceded by "foo". The
- contents of a lookbehind assertion are restricted such that all the
+ does find an occurrence of "bar" that is not preceded by "foo". The
+ contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are sev-
- eral top-level alternatives, they do not all have to have the same
+ eral top-level alternatives, they do not all have to have the same
fixed length. Thus
(?<=bullock|donkey)
@@ -8250,43 +8264,43 @@
(?<!dogs?|cats?)
- causes an error at compile time. Branches that match different length
- strings are permitted only at the top level of a lookbehind assertion.
+ causes an error at compile time. Branches that match different length
+ strings are permitted only at the top level of a lookbehind assertion.
This is an extension compared with Perl, which requires all branches to
match the same length of string. An assertion such as
(?<=ab(c|de))
- is not permitted, because its single top-level branch can match two
- different lengths, but it is acceptable to PCRE2 if rewritten to use
+ is not permitted, because its single top-level branch can match two
+ different lengths, but it is acceptable to PCRE2 if rewritten to use
two top-level branches:
(?<=abc|abde)
- In some cases, the escape sequence \K (see above) can be used instead
+ In some cases, the escape sequence \K (see above) can be used instead
of a lookbehind assertion to get round the fixed-length restriction.
- The implementation of lookbehind assertions is, for each alternative,
- to temporarily move the current position back by the fixed length and
+ The implementation of lookbehind assertions is, for each alternative,
+ to temporarily move the current position back by the fixed length and
then try to match. If there are insufficient characters before the cur-
rent position, the assertion fails.
- In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
- matches a single code unit even in a UTF mode) to appear in lookbehind
- assertions, because it makes it impossible to calculate the length of
- the lookbehind. The \X and \R escapes, which can match different num-
+ In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which
+ matches a single code unit even in a UTF mode) to appear in lookbehind
+ assertions, because it makes it impossible to calculate the length of
+ the lookbehind. The \X and \R escapes, which can match different num-
bers of code units, are never permitted in lookbehinds.
- "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
+ "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
lookbehinds, as long as the called capture group matches a fixed-length
- string. However, recursion, that is, a "subroutine" call into a group
+ string. However, recursion, that is, a "subroutine" call into a group
that is already active, is not supported.
Perl does not support backreferences in lookbehinds. PCRE2 does support
- them, but only if certain conditions are met. The
- PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use
- of (?| in the pattern (it creates duplicate group numbers), and if the
- backreference is by name, the name must be unique. Of course, the ref-
+ them, but only if certain conditions are met. The
+ PCRE2_MATCH_UNSET_BACKREF option must not be set, there must be no use
+ of (?| in the pattern (it creates duplicate group numbers), and if the
+ backreference is by name, the name must be unique. Of course, the ref-
erenced group must itself match a fixed length substring. The following
pattern matches words containing at least two characters that begin and
end with the same character:
@@ -8293,23 +8307,23 @@
\b(\w)\w++(?<=\1)
- Possessive quantifiers can be used in conjunction with lookbehind
+ Possessive quantifiers can be used in conjunction with lookbehind
assertions to specify efficient matching of fixed-length strings at the
end of subject strings. Consider a simple pattern such as
abcd$
- when applied to a long string that does not match. Because matching
- proceeds from left to right, PCRE2 will look for each "a" in the sub-
- ject and then see if what follows matches the rest of the pattern. If
+ when applied to a long string that does not match. Because matching
+ proceeds from left to right, PCRE2 will look for each "a" in the sub-
+ ject and then see if what follows matches the rest of the pattern. If
the pattern is specified as
^.*abcd$
- the initial .* matches the entire string at first, but when this fails
+ the initial .* matches the entire string at first, but when this fails
(because there is no following "a"), it backtracks to match all but the
- last character, then all but the last two characters, and so on. Once
- again the search for "a" covers the entire string, from right to left,
+ last character, then all but the last two characters, and so on. Once
+ again the search for "a" covers the entire string, from right to left,
so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
@@ -8316,8 +8330,8 @@
there can be no backtracking for the .*+ item because of the possessive
quantifier; it can match only the entire string. The subsequent lookbe-
- hind assertion does a single test on the last four characters. If it
- fails, the match fails immediately. For long strings, this approach
+ hind assertion does a single test on the last four characters. If it
+ fails, the match fails immediately. For long strings, this approach
makes a significant difference to the processing time.
Using multiple assertions
@@ -8326,18 +8340,18 @@
(?<=\d{3})(?<!999)foo
- matches "foo" preceded by three digits that are not "999". Notice that
- each of the assertions is applied independently at the same point in
- the subject string. First there is a check that the previous three
- characters are all digits, and then there is a check that the same
+ matches "foo" preceded by three digits that are not "999". Notice that
+ each of the assertions is applied independently at the same point in
+ the subject string. First there is a check that the previous three
+ characters are all digits, and then there is a check that the same
three characters are not "999". This pattern does not match "foo" pre-
- ceded by six characters, the first of which are digits and the last
- three of which are not "999". For example, it doesn't match "123abc-
+ ceded by six characters, the first of which are digits and the last
+ three of which are not "999". For example, it doesn't match "123abc-
foo". A pattern to do that is
(?<=\d{3}...)(?<!999)foo
- This time the first assertion looks at the preceding six characters,
+ This time the first assertion looks at the preceding six characters,
checking that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
@@ -8345,30 +8359,30 @@
(?<=(?<!foo)bar)baz
- matches an occurrence of "baz" that is preceded by "bar" which in turn
+ matches an occurrence of "baz" that is preceded by "bar" which in turn
is not preceded by "foo", while
(?<=\d{3}(?!999)...)foo
- is another pattern that matches "foo" preceded by three digits and any
+ is another pattern that matches "foo" preceded by three digits and any
three characters that are not "999".
SCRIPT RUNS
- In concept, a script run is a sequence of characters that are all from
- the same Unicode script such as Latin or Greek. However, because some
- scripts are commonly used together, and because some diacritical and
- other marks are used with multiple scripts, it is not that simple.
+ In concept, a script run is a sequence of characters that are all from
+ the same Unicode script such as Latin or Greek. However, because some
+ scripts are commonly used together, and because some diacritical and
+ other marks are used with multiple scripts, it is not that simple.
There is a full description of the rules that PCRE2 uses in the section
entitled "Script Runs" in the pcre2unicode documentation.
- If part of a pattern is enclosed between (*script_run: or (*sr: and a
- closing parenthesis, it fails if the sequence of characters that it
- matches are not a script run. After a failure, normal backtracking
- occurs. Script runs can be used to detect spoofing attacks using char-
- acters that look the same, but are from different scripts. The string
- "paypal.com" is an infamous example, where the letters could be a mix-
+ If part of a pattern is enclosed between (*script_run: or (*sr: and a
+ closing parenthesis, it fails if the sequence of characters that it
+ matches are not a script run. After a failure, normal backtracking
+ occurs. Script runs can be used to detect spoofing attacks using char-
+ acters that look the same, but are from different scripts. The string
+ "paypal.com" is an infamous example, where the letters could be a mix-
ture of Latin and Cyrillic. This pattern ensures that the matched char-
acters in a sequence of non-spaces that follow white space are a script
run:
@@ -8375,23 +8389,23 @@
\s+(*sr:\S+)
- To be sure that they are all from the Latin script (for example), a
+ To be sure that they are all from the Latin script (for example), a
lookahead can be used:
\s+(?=\p{Latin})(*sr:\S+)
This works as long as the first character is expected to be a character
- in that script, and not (for example) punctuation, which is allowed
- with any script. If this is not the case, a more creative lookahead is
- needed. For example, if digits, underscore, and dots are permitted at
+ in that script, and not (for example) punctuation, which is allowed
+ with any script. If this is not the case, a more creative lookahead is
+ needed. For example, if digits, underscore, and dots are permitted at
the start:
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
- In many cases, backtracking into a script run pattern fragment is not
- desirable. The script run can employ an atomic group to prevent this.
- Because this is a common requirement, a shorthand notation is provided
+ In many cases, backtracking into a script run pattern fragment is not
+ desirable. The script run can employ an atomic group to prevent this.
+ Because this is a common requirement, a shorthand notation is provided
by (*atomic_script_run: or (*asr:
(*asr:...) is the same as (*sr:(?>...))
@@ -8399,13 +8413,13 @@
Note that the atomic group is inside the script run. Putting it outside
would not prevent backtracking into the script run pattern.
- Support for script runs is not available if PCRE2 is compiled without
+ Support for script runs is not available if PCRE2 is compiled without
Unicode support. A compile-time error is given if any of the above con-
- structs is encountered. Script runs are not supported by the alternate
- matching function, pcre2_dfa_match() because they use the same mecha-
+ structs is encountered. Script runs are not supported by the alternate
+ matching function, pcre2_dfa_match() because they use the same mecha-
nism as capturing parentheses.
- Warning: The (*ACCEPT) control verb (see below) should not be used
+ Warning: The (*ACCEPT) control verb (see below) should not be used
within a script run group, because it causes an immediate exit from the
group, bypassing the script run checking.
@@ -8414,19 +8428,19 @@
It is possible to cause the matching process to obey a pattern fragment
conditionally or to choose between two alternative fragments, depending
- on the result of an assertion, or whether a specific capture group has
+ on the result of an assertion, or whether a specific capture group has
already been matched. The two possible forms of conditional group are:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
- If the condition is satisfied, the yes-pattern is used; otherwise the
- no-pattern (if present) is used. An absent no-pattern is equivalent to
- an empty string (it always matches). If there are more than two alter-
- natives in the group, a compile-time error occurs. Each of the two
- alternatives may itself contain nested groups of any form, including
+ If the condition is satisfied, the yes-pattern is used; otherwise the
+ no-pattern (if present) is used. An absent no-pattern is equivalent to
+ an empty string (it always matches). If there are more than two alter-
+ natives in the group, a compile-time error occurs. Each of the two
+ alternatives may itself contain nested groups of any form, including
conditional groups; the restriction to two alternatives applies only at
- the level of the condition itself. This pattern fragment is an example
+ the level of the condition itself. This pattern fragment is an example
where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
@@ -8433,85 +8447,85 @@
There are five kinds of condition: references to capture groups, refer-
- ences to recursion, two pseudo-conditions called DEFINE and VERSION,
+ ences to recursion, two pseudo-conditions called DEFINE and VERSION,
and assertions.
Checking for a used capture group by number
- If the text between the parentheses consists of a sequence of digits,
- the condition is true if a capture group of that number has previously
- matched. If there is more than one capture group with the same number
- (see the earlier section about duplicate group numbers), the condition
+ If the text between the parentheses consists of a sequence of digits,
+ the condition is true if a capture group of that number has previously
+ matched. If there is more than one capture group with the same number
+ (see the earlier section about duplicate group numbers), the condition
is true if any of them have matched. An alternative notation is to pre-
cede the digits with a plus or minus sign. In this case, the group num-
- ber is relative rather than absolute. The most recently opened capture
- group can be referenced by (?(-1), the next most recent by (?(-2), and
- so on. Inside loops it can also make sense to refer to subsequent
- groups. The next capture group can be referenced as (?(+1), and so on.
- (The value zero in any of these forms is not used; it provokes a com-
+ ber is relative rather than absolute. The most recently opened capture
+ group can be referenced by (?(-1), the next most recent by (?(-2), and
+ so on. Inside loops it can also make sense to refer to subsequent
+ groups. The next capture group can be referenced as (?(+1), and so on.
+ (The value zero in any of these forms is not used; it provokes a com-
pile-time error.)
- Consider the following pattern, which contains non-significant white
- space to make it more readable (assume the PCRE2_EXTENDED option) and
+ Consider the following pattern, which contains non-significant white
+ space to make it more readable (assume the PCRE2_EXTENDED option) and
to divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
- The first part matches an optional opening parenthesis, and if that
+ The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The sec-
- ond part matches one or more characters that are not parentheses. The
- third part is a conditional group that tests whether or not the first
- capture group matched. If it did, that is, if subject started with an
- opening parenthesis, the condition is true, and so the yes-pattern is
- executed and a closing parenthesis is required. Otherwise, since no-
+ ond part matches one or more characters that are not parentheses. The
+ third part is a conditional group that tests whether or not the first
+ capture group matched. If it did, that is, if subject started with an
+ opening parenthesis, the condition is true, and so the yes-pattern is
+ executed and a closing parenthesis is required. Otherwise, since no-
pattern is not present, the conditional group matches nothing. In other
- words, this pattern matches a sequence of non-parentheses, optionally
+ words, this pattern matches a sequence of non-parentheses, optionally
enclosed in parentheses.
- If you were embedding this pattern in a larger one, you could use a
+ If you were embedding this pattern in a larger one, you could use a
relative reference:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
- This makes the fragment independent of the parentheses in the larger
+ This makes the fragment independent of the parentheses in the larger
pattern.
Checking for a used capture group by name
- Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
- used capture group by name. For compatibility with earlier versions of
- PCRE1, which had this facility before Perl, the syntax (?(name)...) is
- also recognized. Note, however, that undelimited names consisting of
- the letter R followed by digits are ambiguous (see the following sec-
+ Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
+ used capture group by name. For compatibility with earlier versions of
+ PCRE1, which had this facility before Perl, the syntax (?(name)...) is
+ also recognized. Note, however, that undelimited names consisting of
+ the letter R followed by digits are ambiguous (see the following sec-
tion). Rewriting the above example to use a named group gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
- If the name used in a condition of this kind is a duplicate, the test
- is applied to all groups of the same name, and is true if any one of
+ If the name used in a condition of this kind is a duplicate, the test
+ is applied to all groups of the same name, and is true if any one of
them has matched.
Checking for pattern recursion
- "Recursion" in this sense refers to any subroutine-like call from one
- part of the pattern to another, whether or not it is actually recur-
- sive. See the sections entitled "Recursive patterns" and "Groups as
+ "Recursion" in this sense refers to any subroutine-like call from one
+ part of the pattern to another, whether or not it is actually recur-
+ sive. See the sections entitled "Recursive patterns" and "Groups as
subroutines" below for details of recursion and subroutine calls.
- If a condition is the string (R), and there is no capture group with
- the name R, the condition is true if matching is currently in a recur-
- sion or subroutine call to the whole pattern or any capture group. If
- digits follow the letter R, and there is no group with that name, the
- condition is true if the most recent call is into a group with the
- given number, which must exist somewhere in the overall pattern. This
+ If a condition is the string (R), and there is no capture group with
+ the name R, the condition is true if matching is currently in a recur-
+ sion or subroutine call to the whole pattern or any capture group. If
+ digits follow the letter R, and there is no group with that name, the
+ condition is true if the most recent call is into a group with the
+ given number, which must exist somewhere in the overall pattern. This
is a contrived example that is equivalent to a+b:
((?(R1)a+|(?1)b))
- However, in both cases, if there is a capture group with a matching
- name, the condition tests for its being set, as described in the sec-
- tion above, instead of testing for recursion. For example, creating a
- group with the name R1 by adding (?<R1>) to the above pattern com-
+ However, in both cases, if there is a capture group with a matching
+ name, the condition tests for its being set, as described in the sec-
+ tion above, instead of testing for recursion. For example, creating a
+ group with the name R1 by adding (?<R1>) to the above pattern com-
pletely changes its meaning.
If a name preceded by ampersand follows the letter R, for example:
@@ -8518,12 +8532,12 @@
(?(R&name)...)
- the condition is true if the most recent recursion is into a group of
+ the condition is true if the most recent recursion is into a group of
that name (which must exist within the pattern).
This condition does not check the entire recursion stack. It tests only
- the current level. If the name used in a condition of this kind is a
- duplicate, the test is applied to all groups of the same name, and is
+ the current level. If the name used in a condition of this kind is a
+ duplicate, the test is applied to all groups of the same name, and is
true if any one of them is the most recent recursion.
At "top level", all these recursion test conditions are false.
@@ -8531,66 +8545,66 @@
Defining capture groups for use by reference only
If the condition is the string (DEFINE), the condition is always false,
- even if there is a group with the name DEFINE. In this case, there may
- be only one alternative in the rest of the conditional group. It is
- always skipped if control reaches this point in the pattern; the idea
+ even if there is a group with the name DEFINE. In this case, there may
+ be only one alternative in the rest of the conditional group. It is
+ always skipped if control reaches this point in the pattern; the idea
of DEFINE is that it can be used to define subroutines that can be ref-
- erenced from elsewhere. (The use of subroutines is described below.)
- For example, a pattern to match an IPv4 address such as
- "192.168.23.245" could be written like this (ignore white space and
+ erenced from elsewhere. (The use of subroutines is described below.)
+ For example, a pattern to match an IPv4 address such as
+ "192.168.23.245" could be written like this (ignore white space and
line breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
- The first part of the pattern is a DEFINE group inside which a another
- group named "byte" is defined. This matches an individual component of
- an IPv4 address (a number less than 256). When matching takes place,
- this part of the pattern is skipped because DEFINE acts like a false
- condition. The rest of the pattern uses references to the named group
- to match the four dot-separated components of an IPv4 address, insist-
+ The first part of the pattern is a DEFINE group inside which a another
+ group named "byte" is defined. This matches an individual component of
+ an IPv4 address (a number less than 256). When matching takes place,
+ this part of the pattern is skipped because DEFINE acts like a false
+ condition. The rest of the pattern uses references to the named group
+ to match the four dot-separated components of an IPv4 address, insist-
ing on a word boundary at each end.
Checking the PCRE2 version
- Programs that link with a PCRE2 library can check the version by call-
- ing pcre2_config() with appropriate arguments. Users of applications
- that do not have access to the underlying code cannot do this. A spe-
- cial "condition" called VERSION exists to allow such users to discover
+ Programs that link with a PCRE2 library can check the version by call-
+ ing pcre2_config() with appropriate arguments. Users of applications
+ that do not have access to the underlying code cannot do this. A spe-
+ cial "condition" called VERSION exists to allow such users to discover
which version of PCRE2 they are dealing with by using this condition to
- match a string such as "yesno". VERSION must be followed either by "="
+ match a string such as "yesno". VERSION must be followed either by "="
or ">=" and a version number. For example:
(?(VERSION>=10.4)yes|no)
- This pattern matches "yes" if the PCRE2 version is greater or equal to
- 10.4, or "no" otherwise. The fractional part of the version number may
+ This pattern matches "yes" if the PCRE2 version is greater or equal to
+ 10.4, or "no" otherwise. The fractional part of the version number may
not contain more than two digits.
Assertion conditions
- If the condition is not in any of the above formats, it must be a
- parenthesized assertion. This may be a positive or negative lookahead
- or lookbehind assertion. Consider this pattern, again containing non-
- significant white space, and with the two alternatives on the second
+ If the condition is not in any of the above formats, it must be a
+ parenthesized assertion. This may be a positive or negative lookahead
+ or lookbehind assertion. Consider this pattern, again containing non-
+ significant white space, and with the two alternatives on the second
line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
- The condition is a positive lookahead assertion that matches an
- optional sequence of non-letters followed by a letter. In other words,
- it tests for the presence of at least one letter in the subject. If a
- letter is found, the subject is matched against the first alternative;
- otherwise it is matched against the second. This pattern matches
- strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
+ The condition is a positive lookahead assertion that matches an
+ optional sequence of non-letters followed by a letter. In other words,
+ it tests for the presence of at least one letter in the subject. If a
+ letter is found, the subject is matched against the first alternative;
+ otherwise it is matched against the second. This pattern matches
+ strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
When an assertion that is a condition contains capture groups, any cap-
- turing that occurs in a matching branch is retained afterwards, for
- both positive and negative assertions, because matching always contin-
- ues after the assertion, whether it succeeds or fails. (Compare non-
- conditional assertions, for which captures are retained only for posi-
+ turing that occurs in a matching branch is retained afterwards, for
+ both positive and negative assertions, because matching always contin-
+ ues after the assertion, whether it succeeds or fails. (Compare non-
+ conditional assertions, for which captures are retained only for posi-
tive assertions that succeed.)
@@ -8597,44 +8611,44 @@
COMMENTS
There are two ways of including comments in patterns that are processed
- by PCRE2. In both cases, the start of the comment must not be in a
- character class, nor in the middle of any other sequence of related
- characters such as (?: or a group name or number. The characters that
+ by PCRE2. In both cases, the start of the comment must not be in a
+ character class, nor in the middle of any other sequence of related
+ characters such as (?: or a group name or number. The characters that
make up a comment play no part in the pattern matching.
- The sequence (?# marks the start of a comment that continues up to the
- next closing parenthesis. Nested parentheses are not permitted. If the
- PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped #
- character also introduces a comment, which in this case continues to
- immediately after the next newline character or character sequence in
+ The sequence (?# marks the start of a comment that continues up to the
+ next closing parenthesis. Nested parentheses are not permitted. If the
+ PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped #
+ character also introduces a comment, which in this case continues to
+ immediately after the next newline character or character sequence in
the pattern. Which characters are interpreted as newlines is controlled
- by an option passed to the compiling function or by a special sequence
+ by an option passed to the compiling function or by a special sequence
at the start of the pattern, as described in the section entitled "New-
line conventions" above. Note that the end of this type of comment is a
- literal newline sequence in the pattern; escape sequences that happen
+ literal newline sequence in the pattern; escape sequences that happen
to represent a newline do not count. For example, consider this pattern
- when PCRE2_EXTENDED is set, and the default newline convention (a sin-
+ when PCRE2_EXTENDED is set, and the default newline convention (a sin-
gle linefeed character) is in force:
abc #comment \n still comment
- On encountering the # character, pcre2_compile() skips along, looking
- for a newline in the pattern. The sequence \n is still literal at this
- stage, so it does not terminate the comment. Only an actual character
+ On encountering the # character, pcre2_compile() skips along, looking
+ for a newline in the pattern. The sequence \n is still literal at this
+ stage, so it does not terminate the comment. Only an actual character
with the code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
- Consider the problem of matching a string in parentheses, allowing for
- unlimited nested parentheses. Without the use of recursion, the best
- that can be done is to use a pattern that matches up to some fixed
- depth of nesting. It is not possible to handle an arbitrary nesting
+ Consider the problem of matching a string in parentheses, allowing for
+ unlimited nested parentheses. Without the use of recursion, the best
+ that can be done is to use a pattern that matches up to some fixed
+ depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular expres-
- sions to recurse (amongst other things). It does this by interpolating
- Perl code in the expression at run time, and the code can refer to the
+ sions to recurse (amongst other things). It does this by interpolating
+ Perl code in the expression at run time, and the code can refer to the
expression itself. A Perl pattern using code interpolation to solve the
parentheses problem can be created like this:
@@ -8644,66 +8658,66 @@
refers recursively to the pattern in which it appears.
Obviously, PCRE2 cannot support the interpolation of Perl code.
- Instead, it supports special syntax for recursion of the entire pat-
+ Instead, it supports special syntax for recursion of the entire pat-
tern, and also for individual capture group recursion. After its intro-
- duction in PCRE1 and Python, this kind of recursion was subsequently
+ duction in PCRE1 and Python, this kind of recursion was subsequently
introduced into Perl at release 5.10.
- A special item that consists of (? followed by a number greater than
- zero and a closing parenthesis is a recursive subroutine call of the
- capture group of the given number, provided that it occurs inside that
- group. (If not, it is a non-recursive subroutine call, which is
- described in the next section.) The special item (?R) or (?0) is a
+ A special item that consists of (? followed by a number greater than
+ zero and a closing parenthesis is a recursive subroutine call of the
+ capture group of the given number, provided that it occurs inside that
+ group. (If not, it is a non-recursive subroutine call, which is
+ described in the next section.) The special item (?R) or (?0) is a
recursive call of the entire regular expression.
- This PCRE2 pattern solves the nested parentheses problem (assume the
+ This PCRE2 pattern solves the nested parentheses problem (assume the
PCRE2_EXTENDED option is set so that white space is ignored):
\( ( [^()]++ | (?R) )* \)
- First it matches an opening parenthesis. Then it matches any number of
- substrings which can either be a sequence of non-parentheses, or a
- recursive match of the pattern itself (that is, a correctly parenthe-
+ First it matches an opening parenthesis. Then it matches any number of
+ substrings which can either be a sequence of non-parentheses, or a
+ recursive match of the pattern itself (that is, a correctly parenthe-
sized substring). Finally there is a closing parenthesis. Note the use
of a possessive quantifier to avoid backtracking into sequences of non-
parentheses.
- If this were part of a larger pattern, you would not want to recurse
+ If this were part of a larger pattern, you would not want to recurse
the entire pattern, so instead you could use this:
( \( ( [^()]++ | (?1) )* \) )
- We have put the pattern into parentheses, and caused the recursion to
+ We have put the pattern into parentheses, and caused the recursion to
refer to them instead of the whole pattern.
- In a larger pattern, keeping track of parenthesis numbers can be
- tricky. This is made easier by the use of relative references. Instead
+ In a larger pattern, keeping track of parenthesis numbers can be
+ tricky. This is made easier by the use of relative references. Instead
of (?1) in the pattern above you can write (?-2) to refer to the second
- most recently opened parentheses preceding the recursion. In other
- words, a negative number counts capturing parentheses leftwards from
+ most recently opened parentheses preceding the recursion. In other
+ words, a negative number counts capturing parentheses leftwards from
the point at which it is encountered.
- Be aware however, that if duplicate capture group numbers are in use,
- relative references refer to the earliest group with the appropriate
+ Be aware however, that if duplicate capture group numbers are in use,
+ relative references refer to the earliest group with the appropriate
number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
The first two capture groups (a) and (b) are both numbered 1, and group
- (c) is number 2. When the reference (?-2) is encountered, the second
- most recently opened parentheses has the number 1, but it is the first
+ (c) is number 2. When the reference (?-2) is encountered, the second
+ most recently opened parentheses has the number 1, but it is the first
such group (the (a) group) to which the recursion refers. This would be
- the same if an absolute reference (?1) was used. In other words, rela-
+ the same if an absolute reference (?1) was used. In other words, rela-
tive references are just a shorthand for computing a group number.
- It is also possible to refer to subsequent capture groups, by writing
- references such as (?+2). However, these cannot be recursive because
- the reference is not inside the parentheses that are referenced. They
- are always non-recursive subroutine calls, as described in the next
+ It is also possible to refer to subsequent capture groups, by writing
+ references such as (?+2). However, these cannot be recursive because
+ the reference is not inside the parentheses that are referenced. They
+ are always non-recursive subroutine calls, as described in the next
section.
- An alternative approach is to use named parentheses. The Perl syntax
- for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup-
+ An alternative approach is to use named parentheses. The Perl syntax
+ for this is (?&name); PCRE1's earlier syntax (?P>name) is also sup-
ported. We could rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
@@ -8712,40 +8726,40 @@
used.
The example pattern that we have been looking at contains nested unlim-
- ited repeats, and so the use of a possessive quantifier for matching
- strings of non-parentheses is important when applying the pattern to
+ ited repeats, and so the use of a possessive quantifier for matching
+ strings of non-parentheses is important when applying the pattern to
strings that do not match. For example, when this pattern is applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
- it yields "no match" quickly. However, if a possessive quantifier is
- not used, the match runs for a very long time indeed because there are
- so many different ways the + and * repeats can carve up the subject,
+ it yields "no match" quickly. However, if a possessive quantifier is
+ not used, the match runs for a very long time indeed because there are
+ so many different ways the + and * repeats can carve up the subject,
and all have to be tested before failure can be reported.
- At the end of a match, the values of capturing parentheses are those
- from the outermost level. If you want to obtain intermediate values, a
+ At the end of a match, the values of capturing parentheses are those
+ from the outermost level. If you want to obtain intermediate values, a
callout function can be used (see below and the pcre2callout documenta-
tion). If the pattern above is matched against
(ab(cd)ef)
- the value for the inner capturing parentheses (numbered 2) is "ef",
- which is the last value taken on at the top level. If a capture group
- is not matched at the top level, its final captured value is unset,
- even if it was (temporarily) set at a deeper level during the matching
+ the value for the inner capturing parentheses (numbered 2) is "ef",
+ which is the last value taken on at the top level. If a capture group
+ is not matched at the top level, its final captured value is unset,
+ even if it was (temporarily) set at a deeper level during the matching
process.
- Do not confuse the (?R) item with the condition (R), which tests for
- recursion. Consider this pattern, which matches text in angle brack-
- ets, allowing for arbitrary nesting. Only digits are allowed in nested
- brackets (that is, when recursing), whereas any characters are permit-
+ Do not confuse the (?R) item with the condition (R), which tests for
+ recursion. Consider this pattern, which matches text in angle brack-
+ ets, allowing for arbitrary nesting. Only digits are allowed in nested
+ brackets (that is, when recursing), whereas any characters are permit-
ted at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
- In this pattern, (?(R) is the start of a conditional group, with two
- different alternatives for the recursive and non-recursive cases. The
+ In this pattern, (?(R) is the start of a conditional group, with two
+ different alternatives for the recursive and non-recursive cases. The
(?R) item is the actual recursive call.
Differences in recursion processing between PCRE2 and Perl
@@ -8752,66 +8766,66 @@
Some former differences between PCRE2 and Perl no longer exist.
- Before release 10.30, recursion processing in PCRE2 differed from Perl
- in that a recursive subroutine call was always treated as an atomic
- group. That is, once it had matched some of the subject string, it was
- never re-entered, even if it contained untried alternatives and there
- was a subsequent matching failure. (Historical note: PCRE implemented
+ Before release 10.30, recursion processing in PCRE2 differed from Perl
+ in that a recursive subroutine call was always treated as an atomic
+ group. That is, once it had matched some of the subject string, it was
+ never re-entered, even if it contained untried alternatives and there
+ was a subsequent matching failure. (Historical note: PCRE implemented
recursion before Perl did.)
- Starting with release 10.30, recursive subroutine calls are no longer
+ Starting with release 10.30, recursive subroutine calls are no longer
treated as atomic. That is, they can be re-entered to try unused alter-
- natives if there is a matching failure later in the pattern. This is
- now compatible with the way Perl works. If you want a subroutine call
+ natives if there is a matching failure later in the pattern. This is
+ now compatible with the way Perl works. If you want a subroutine call
to be atomic, you must explicitly enclose it in an atomic group.
- Supporting backtracking into recursions simplifies certain types of
+ Supporting backtracking into recursions simplifies certain types of
recursive pattern. For example, this pattern matches palindromic
strings:
^((.)(?1)\2|.?)$
- The second branch in the group matches a single central character in
- the palindrome when there are an odd number of characters, or nothing
- when there are an even number of characters, but in order to work it
- has to be able to try the second case when the rest of the pattern
+ The second branch in the group matches a single central character in
+ the palindrome when there are an odd number of characters, or nothing
+ when there are an even number of characters, but in order to work it
+ has to be able to try the second case when the rest of the pattern
match fails. If you want to match typical palindromic phrases, the pat-
- tern has to ignore all non-word characters, which can be done like
+ tern has to ignore all non-word characters, which can be done like
this:
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
- If run with the PCRE2_CASELESS option, this pattern matches phrases
- such as "A man, a plan, a canal: Panama!". Note the use of the posses-
- sive quantifier *+ to avoid backtracking into sequences of non-word
+ If run with the PCRE2_CASELESS option, this pattern matches phrases
+ such as "A man, a plan, a canal: Panama!". Note the use of the posses-
+ sive quantifier *+ to avoid backtracking into sequences of non-word
characters. Without this, PCRE2 takes a great deal longer (ten times or
- more) to match typical phrases, and Perl takes so long that you think
+ more) to match typical phrases, and Perl takes so long that you think
it has gone into a loop.
- Another way in which PCRE2 and Perl used to differ in their recursion
- processing is in the handling of captured values. Formerly in Perl,
- when a group was called recursively or as a subroutine (see the next
+ Another way in which PCRE2 and Perl used to differ in their recursion
+ processing is in the handling of captured values. Formerly in Perl,
+ when a group was called recursively or as a subroutine (see the next
section), it had no access to any values that were captured outside the
- recursion, whereas in PCRE2 these values can be referenced. Consider
+ recursion, whereas in PCRE2 these values can be referenced. Consider
this pattern:
^(.)(\1|a(?2))
- This pattern matches "bab". The first capturing parentheses match "b",
+ This pattern matches "bab". The first capturing parentheses match "b",
then in the second group, when the backreference \1 fails to match "b",
the second alternative matches "a" and then recurses. In the recursion,
- \1 does now match "b" and so the whole match succeeds. This match used
+ \1 does now match "b" and so the whole match succeeds. This match used
to fail in Perl, but in later versions (I tried 5.024) it now works.
GROUPS AS SUBROUTINES
- If the syntax for a recursive group call (either by number or by name)
- is used outside the parentheses to which it refers, it operates a bit
- like a subroutine in a programming language. More accurately, PCRE2
+ If the syntax for a recursive group call (either by number or by name)
+ is used outside the parentheses to which it refers, it operates a bit
+ like a subroutine in a programming language. More accurately, PCRE2
treats the referenced group as an independent subpattern which it tries
- to match at the current matching position. The called group may be
- defined before or after the reference. A numbered reference can be
+ to match at the current matching position. The called group may be
+ defined before or after the reference. A numbered reference can be
absolute or relative, as in these examples:
(...(absolute)...)...(?2)...
@@ -8822,30 +8836,30 @@
(sens|respons)e and \1ibility
- matches "sense and sensibility" and "response and responsibility", but
+ matches "sense and sensibility" and "response and responsibility", but
not "sense and responsibility". If instead the pattern
(sens|respons)e and (?1)ibility
- is used, it does match "sense and responsibility" as well as the other
- two strings. Another example is given in the discussion of DEFINE
+ is used, it does match "sense and responsibility" as well as the other
+ two strings. Another example is given in the discussion of DEFINE
above.
- Like recursions, subroutine calls used to be treated as atomic, but
- this changed at PCRE2 release 10.30, so backtracking into subroutine
- calls can now occur. However, any capturing parentheses that are set
+ Like recursions, subroutine calls used to be treated as atomic, but
+ this changed at PCRE2 release 10.30, so backtracking into subroutine
+ calls can now occur. However, any capturing parentheses that are set
during the subroutine call revert to their previous values afterwards.
- Processing options such as case-independence are fixed when a group is
- defined, so if it is used as a subroutine, such options cannot be
+ Processing options such as case-independence are fixed when a group is
+ defined, so if it is used as a subroutine, such options cannot be
changed for different calls. For example, consider this pattern:
(abc)(?i:(?-1))
- It matches "abcabc". It does not match "abcABC" because the change of
+ It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called group.
- The behaviour of backtracking control verbs in groups when called as
+ The behaviour of backtracking control verbs in groups when called as
subroutines is described in the section entitled "Backtracking verbs in
subroutines" below.
@@ -8852,22 +8866,22 @@
ONIGURUMA SUBROUTINE SYNTAX
- For compatibility with Oniguruma, the non-Perl syntax \g followed by a
+ For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
- an alternative syntax for calling a group as a subroutine, possibly
- recursively. Here are two of the examples used above, rewritten using
+ an alternative syntax for calling a group as a subroutine, possibly
+ recursively. Here are two of the examples used above, rewritten using
this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
- PCRE2 supports an extension to Oniguruma: if a number is preceded by a
+ PCRE2 supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
(abc)(?i:\g<-1>)
- Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
- synonymous. The former is a backreference; the latter is a subroutine
+ Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
+ synonymous. The former is a backreference; the latter is a subroutine
call.
@@ -8874,54 +8888,54 @@
CALLOUTS
Perl has a feature whereby using the sequence (?{...}) causes arbitrary
- Perl code to be obeyed in the middle of matching a regular expression.
+ Perl code to be obeyed in the middle of matching a regular expression.
This makes it possible, amongst other things, to extract different sub-
strings that match the same pair of parentheses when there is a repeti-
tion.
- PCRE2 provides a similar feature, but of course it cannot obey arbi-
- trary Perl code. The feature is called "callout". The caller of PCRE2
- provides an external function by putting its entry point in a match
- context using the function pcre2_set_callout(), and then passing that
- context to pcre2_match() or pcre2_dfa_match(). If no match context is
+ PCRE2 provides a similar feature, but of course it cannot obey arbi-
+ trary Perl code. The feature is called "callout". The caller of PCRE2
+ provides an external function by putting its entry point in a match
+ context using the function pcre2_set_callout(), and then passing that
+ context to pcre2_match() or pcre2_dfa_match(). If no match context is
passed, or if the callout entry point is set to NULL, callouts are dis-
abled.
- Within a regular expression, (?C<arg>) indicates a point at which the
- external function is to be called. There are two kinds of callout:
- those with a numerical argument and those with a string argument. (?C)
- on its own with no argument is treated as (?C0). A numerical argument
- allows the application to distinguish between different callouts.
- String arguments were added for release 10.20 to make it possible for
- script languages that use PCRE2 to embed short scripts within patterns
+ Within a regular expression, (?C<arg>) indicates a point at which the
+ external function is to be called. There are two kinds of callout:
+ those with a numerical argument and those with a string argument. (?C)
+ on its own with no argument is treated as (?C0). A numerical argument
+ allows the application to distinguish between different callouts.
+ String arguments were added for release 10.20 to make it possible for
+ script languages that use PCRE2 to embed short scripts within patterns
in a similar way to Perl.
During matching, when PCRE2 reaches a callout point, the external func-
- tion is called. It is provided with the number or string argument of
- the callout, the position in the pattern, and one item of data that is
+ tion is called. It is provided with the number or string argument of
+ the callout, the position in the pattern, and one item of data that is
also set in the match block. The callout function may cause matching to
proceed, to backtrack, or to fail.
- By default, PCRE2 implements a number of optimizations at matching
- time, and one side-effect is that sometimes callouts are skipped. If
- you need all possible callouts to happen, you need to set options that
- disable the relevant optimizations. More details, including a complete
- description of the programming interface to the callout function, are
+ By default, PCRE2 implements a number of optimizations at matching
+ time, and one side-effect is that sometimes callouts are skipped. If
+ you need all possible callouts to happen, you need to set options that
+ disable the relevant optimizations. More details, including a complete
+ description of the programming interface to the callout function, are
given in the pcre2callout documentation.
Callouts with numerical arguments
- If you just want to have a means of identifying different callout
- points, put a number less than 256 after the letter C. For example,
+ If you just want to have a means of identifying different callout
+ points, put a number less than 256 after the letter C. For example,
this pattern has two callout points:
(?C1)abc(?C2)def
- If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
- callouts are automatically installed before each item in the pattern.
- They are all numbered 255. If there is a conditional group in the pat-
+ If the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
+ callouts are automatically installed before each item in the pattern.
+ They are all numbered 255. If there is a conditional group in the pat-
tern whose condition is an assertion, an additional callout is inserted
- just before the condition. An explicit callout may also be set at this
+ just before the condition. An explicit callout may also be set at this
position, as in this example:
(?(?C9)(?=a)abc|def)
@@ -8931,62 +8945,62 @@
Callouts with string arguments
- A delimited string may be used instead of a number as a callout argu-
- ment. The starting delimiter must be one of ` ' " ^ % # $ { and the
+ A delimited string may be used instead of a number as a callout argu-
+ ment. The starting delimiter must be one of ` ' " ^ % # $ { and the
ending delimiter is the same as the start, except for {, where the end-
- ing delimiter is }. If the ending delimiter is needed within the
+ ing delimiter is }. If the ending delimiter is needed within the
string, it must be doubled. For example:
(?C'ab ''c'' d')xyz(?C{any text})pqr
- The doubling is removed before the string is passed to the callout
+ The doubling is removed before the string is passed to the callout
function.
BACKTRACKING CONTROL
- There are a number of special "Backtracking Control Verbs" (to use
- Perl's terminology) that modify the behaviour of backtracking during
- matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
+ There are a number of special "Backtracking Control Verbs" (to use
+ Perl's terminology) that modify the behaviour of backtracking during
+ matching. They are generally of the form (*VERB) or (*VERB:NAME). Some
verbs take either form, and may behave differently depending on whether
- or not a name argument is present. The names are not required to be
+ or not a name argument is present. The names are not required to be
unique within the pattern.
- By default, for compatibility with Perl, a name is any sequence of
+ By default, for compatibility with Perl, a name is any sequence of
characters that does not include a closing parenthesis. The name is not
- processed in any way, and it is not possible to include a closing
- parenthesis in the name. This can be changed by setting the
- PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati-
+ processed in any way, and it is not possible to include a closing
+ parenthesis in the name. This can be changed by setting the
+ PCRE2_ALT_VERBNAMES option, but the result is no longer Perl-compati-
ble.
- When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
- verb names and only an unescaped closing parenthesis terminates the
- name. However, the only backslash items that are permitted are \Q, \E,
- and sequences such as \x{100} that define character code points. Char-
+ When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to
+ verb names and only an unescaped closing parenthesis terminates the
+ name. However, the only backslash items that are permitted are \Q, \E,
+ and sequences such as \x{100} that define character code points. Char-
acter type escapes such as \d are faulted.
A closing parenthesis can be included in a name either as \) or between
- \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
+ \Q and \E. In addition to backslash processing, if the PCRE2_EXTENDED
or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb
names is skipped, and #-comments are recognized, exactly as in the rest
- of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect
+ of the pattern. PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect
verb names unless PCRE2_ALT_VERBNAMES is also set.
- The maximum length of a name is 255 in the 8-bit library and 65535 in
- the 16-bit and 32-bit libraries. If the name is empty, that is, if the
- closing parenthesis immediately follows the colon, the effect is as if
+ The maximum length of a name is 255 in the 8-bit library and 65535 in
+ the 16-bit and 32-bit libraries. If the name is empty, that is, if the
+ closing parenthesis immediately follows the colon, the effect is as if
the colon were not there. Any number of these verbs may occur in a pat-
tern. Except for (*ACCEPT), they may not be quantified.
- Since these verbs are specifically related to backtracking, most of
- them can be used only when the pattern is to be matched using the tra-
+ Since these verbs are specifically related to backtracking, most of
+ them can be used only when the pattern is to be matched using the tra-
ditional matching function, because that uses a backtracking algorithm.
- With the exception of (*FAIL), which behaves like a failing negative
+ With the exception of (*FAIL), which behaves like a failing negative
assertion, the backtracking control verbs cause an error if encountered
by the DFA matching function.
- The behaviour of these verbs in repeated groups, assertions, and in
- capture groups called as subroutines (whether or not recursively) is
+ The behaviour of these verbs in repeated groups, assertions, and in
+ capture groups called as subroutines (whether or not recursively) is
documented below.
Optimizations that affect backtracking verbs
@@ -8993,16 +9007,16 @@
PCRE2 contains some optimizations that are used to speed up matching by
running some checks at the start of each match attempt. For example, it
- may know the minimum length of matching subject, or that a particular
+ may know the minimum length of matching subject, or that a particular
character must be present. When one of these optimizations bypasses the
- running of a match, any included backtracking verbs will not, of
+ running of a match, any included backtracking verbs will not, of
course, be processed. You can suppress the start-of-match optimizations
- by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
- pile(), or by starting the pattern with (*NO_START_OPT). There is more
+ by setting the PCRE2_NO_START_OPTIMIZE option when calling pcre2_com-
+ pile(), or by starting the pattern with (*NO_START_OPT). There is more
discussion of this option in the section entitled "Compiling a pattern"
in the pcre2api documentation.
- Experiments with Perl suggest that it too has similar optimizations,
+ Experiments with Perl suggest that it too has similar optimizations,
and like PCRE2, turning them off can change the result of a match.
Verbs that act immediately
@@ -9011,77 +9025,77 @@
(*ACCEPT) or (*ACCEPT:NAME)
- This verb causes the match to end successfully, skipping the remainder
- of the pattern. However, when it is inside a capture group that is
+ This verb causes the match to end successfully, skipping the remainder
+ of the pattern. However, when it is inside a capture group that is
called as a subroutine, only that group is ended successfully. Matching
then continues at the outer level. If (*ACCEPT) in triggered in a posi-
- tive assertion, the assertion succeeds; in a negative assertion, the
+ tive assertion, the assertion succeeds; in a negative assertion, the
assertion fails.
- If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
+ If (*ACCEPT) is inside capturing parentheses, the data so far is cap-
tured. For example:
A((?:A|B(*ACCEPT)|C)D)
- This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
+ This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
tured by the outer parentheses.
- (*ACCEPT) is the only backtracking verb that is allowed to be quanti-
- fied because an ungreedy quantification with a minimum of zero acts
+ (*ACCEPT) is the only backtracking verb that is allowed to be quanti-
+ fied because an ungreedy quantification with a minimum of zero acts
only when a backtrack happens. Consider, for example,
(A(*ACCEPT)??B)C
- where A, B, and C may be complex expressions. After matching "A", the
- matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT)
- is triggered and the match succeeds. In both cases, all but C is cap-
- tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a
+ where A, B, and C may be complex expressions. After matching "A", the
+ matcher processes "BC"; if that fails, causing a backtrack, (*ACCEPT)
+ is triggered and the match succeeds. In both cases, all but C is cap-
+ tured. Whereas (*COMMIT) (see below) means "fail on backtrack", a
repeated (*ACCEPT) of this type means "succeed on backtrack".
- Warning: (*ACCEPT) should not be used within a script run group,
- because it causes an immediate exit from the group, bypassing the
+ Warning: (*ACCEPT) should not be used within a script run group,
+ because it causes an immediate exit from the group, bypassing the
script run checking.
(*FAIL) or (*FAIL:NAME)
- This verb causes a matching failure, forcing backtracking to occur. It
- may be abbreviated to (*F). It is equivalent to (?!) but easier to
+ This verb causes a matching failure, forcing backtracking to occur. It
+ may be abbreviated to (*F). It is equivalent to (?!) but easier to
read. The Perl documentation notes that it is probably useful only when
combined with (?{}) or (??{}). Those are, of course, Perl features that
- are not present in PCRE2. The nearest equivalent is the callout fea-
+ are not present in PCRE2. The nearest equivalent is the callout fea-
ture, as for example in this pattern:
a+(?C)(*FAIL)
- A match with the string "aaaa" always fails, but the callout is taken
+ A match with the string "aaaa" always fails, but the callout is taken
before each backtrack happens (in this example, 10 times).
- (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as
+ (*ACCEPT:NAME) and (*FAIL:NAME) behave the same as
(*MARK:NAME)(*ACCEPT) and (*MARK:NAME)(*FAIL), respectively, that is, a
(*MARK) is recorded just before the verb acts.
Recording which path was taken
- There is one verb whose main purpose is to track how a match was
- arrived at, though it also has a secondary use in conjunction with
+ There is one verb whose main purpose is to track how a match was
+ arrived at, though it also has a secondary use in conjunction with
advancing the match starting point (see (*SKIP) below).
(*MARK:NAME) or (*:NAME)
- A name is always required with this verb. For all the other backtrack-
+ A name is always required with this verb. For all the other backtrack-
ing control verbs, a NAME argument is optional.
- When a match succeeds, the name of the last-encountered mark name on
+ When a match succeeds, the name of the last-encountered mark name on
the matching path is passed back to the caller as described in the sec-
tion entitled "Other information about the match" in the pcre2api docu-
- mentation. This applies to all instances of (*MARK) and other verbs,
+ mentation. This applies to all instances of (*MARK) and other verbs,
including those inside assertions and atomic groups. However, there are
- differences in those cases when (*MARK) is used in conjunction with
+ differences in those cases when (*MARK) is used in conjunction with
(*SKIP) as described below.
- The mark name that was last encountered on the matching path is passed
- back. A verb without a NAME argument is ignored for this purpose. Here
- is an example of pcre2test output, where the "mark" modifier requests
+ The mark name that was last encountered on the matching path is passed
+ back. A verb without a NAME argument is ignored for this purpose. Here
+ is an example of pcre2test output, where the "mark" modifier requests
the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
@@ -9093,16 +9107,16 @@
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this exam-
- ple it indicates which of the two alternatives matched. This is a more
- efficient way of obtaining this information than putting each alterna-
+ ple it indicates which of the two alternatives matched. This is a more
+ efficient way of obtaining this information than putting each alterna-
tive in its own capturing parentheses.
- If a verb with a name is encountered in a positive assertion that is
- true, the name is recorded and passed back if it is the last-encoun-
+ If a verb with a name is encountered in a positive assertion that is
+ true, the name is recorded and passed back if it is the last-encoun-
tered. This does not happen for negative assertions or failing positive
assertions.
- After a partial match or a failed match, the last encountered name in
+ After a partial match or a failed match, the last encountered name in
the entire match process is returned. For example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
@@ -9109,38 +9123,38 @@
data> XP
No match, mark = B
- Note that in this unanchored example the mark is retained from the
+ Note that in this unanchored example the mark is retained from the
match attempt that started at the letter "X" in the subject. Subsequent
match attempts starting at "P" and then with an empty string do not get
as far as the (*MARK) item, but nevertheless do not reset it.
- If you are interested in (*MARK) values after failed matches, you
- should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to
+ If you are interested in (*MARK) values after failed matches, you
+ should probably set the PCRE2_NO_START_OPTIMIZE option (see above) to
ensure that the match is always attempted.
Verbs that act after backtracking
The following verbs do nothing when they are encountered. Matching con-
- tinues with what follows, but if there is a subsequent match failure,
- causing a backtrack to the verb, a failure is forced. That is, back-
- tracking cannot pass to the left of the verb. However, when one of
+ tinues with what follows, but if there is a subsequent match failure,
+ causing a backtrack to the verb, a failure is forced. That is, back-
+ tracking cannot pass to the left of the verb. However, when one of
these verbs appears inside an atomic group or in a lookaround assertion
- that is true, its effect is confined to that group, because once the
- group has been matched, there is never any backtracking into it. Back-
+ that is true, its effect is confined to that group, because once the
+ group has been matched, there is never any backtracking into it. Back-
tracking from beyond an assertion or an atomic group ignores the entire
group, and seeks a preceding backtracking point.
- These verbs differ in exactly what kind of failure occurs when back-
- tracking reaches them. The behaviour described below is what happens
- when the verb is not in a subroutine or an assertion. Subsequent sec-
+ These verbs differ in exactly what kind of failure occurs when back-
+ tracking reaches them. The behaviour described below is what happens
+ when the verb is not in a subroutine or an assertion. Subsequent sec-
tions cover these special cases.
(*COMMIT) or (*COMMIT:NAME)
- This verb causes the whole match to fail outright if there is a later
+ This verb causes the whole match to fail outright if there is a later
matching failure that causes backtracking to reach it. Even if the pat-
- tern is unanchored, no further attempts to find a match by advancing
- the starting point take place. If (*COMMIT) is the only backtracking
+ tern is unanchored, no further attempts to find a match by advancing
+ the starting point take place. If (*COMMIT) is the only backtracking
verb that is encountered, once it has been passed pcre2_match() is com-
mitted to finding a match at the current starting point, or not at all.
For example:
@@ -9147,22 +9161,22 @@
a+(*COMMIT)b
- This matches "xxaab" but not "aacaab". It can be thought of as a kind
+ This matches "xxaab" but not "aacaab". It can be thought of as a kind
of dynamic anchor, or "I've started, so I must finish."
- The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM-
- MIT). It is like (*MARK:NAME) in that the name is remembered for pass-
- ing back to the caller. However, (*SKIP:NAME) searches only for names
+ The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COM-
+ MIT). It is like (*MARK:NAME) in that the name is remembered for pass-
+ ing back to the caller. However, (*SKIP:NAME) searches only for names
that are set with (*MARK), ignoring those set by any of the other back-
tracking verbs.
- If there is more than one backtracking verb in a pattern, a different
- one that follows (*COMMIT) may be triggered first, so merely passing
+ If there is more than one backtracking verb in a pattern, a different
+ one that follows (*COMMIT) may be triggered first, so merely passing
(*COMMIT) during a match does not always guarantee that a match must be
at this starting point.
- Note that (*COMMIT) at the start of a pattern is not the same as an
- anchor, unless PCRE2's start-of-match optimizations are turned off, as
+ Note that (*COMMIT) at the start of a pattern is not the same as an
+ anchor, unless PCRE2's start-of-match optimizations are turned off, as
shown in this output from pcre2test:
re> /(*COMMIT)abc/
@@ -9173,55 +9187,56 @@
data> xyzabc
No match
- For the first pattern, PCRE2 knows that any match must start with "a",
- so the optimization skips along the subject to "a" before applying the
- pattern to the first set of data. The match attempt then succeeds. The
- second pattern disables the optimization that skips along to the first
- character. The pattern is now applied starting at "x", and so the
- (*COMMIT) causes the match to fail without trying any other starting
+ For the first pattern, PCRE2 knows that any match must start with "a",
+ so the optimization skips along the subject to "a" before applying the
+ pattern to the first set of data. The match attempt then succeeds. The
+ second pattern disables the optimization that skips along to the first
+ character. The pattern is now applied starting at "x", and so the
+ (*COMMIT) causes the match to fail without trying any other starting
points.
(*PRUNE) or (*PRUNE:NAME)
- This verb causes the match to fail at the current starting position in
+ This verb causes the match to fail at the current starting position in
the subject if there is a later matching failure that causes backtrack-
- ing to reach it. If the pattern is unanchored, the normal "bumpalong"
- advance to the next starting character then happens. Backtracking can
- occur as usual to the left of (*PRUNE), before it is reached, or when
- matching to the right of (*PRUNE), but if there is no match to the
- right, backtracking cannot cross (*PRUNE). In simple cases, the use of
- (*PRUNE) is just an alternative to an atomic group or possessive quan-
+ ing to reach it. If the pattern is unanchored, the normal "bumpalong"
+ advance to the next starting character then happens. Backtracking can
+ occur as usual to the left of (*PRUNE), before it is reached, or when
+ matching to the right of (*PRUNE), but if there is no match to the
+ right, backtracking cannot cross (*PRUNE). In simple cases, the use of
+ (*PRUNE) is just an alternative to an atomic group or possessive quan-
tifier, but there are some uses of (*PRUNE) that cannot be expressed in
- any other way. In an anchored pattern (*PRUNE) has the same effect as
+ any other way. In an anchored pattern (*PRUNE) has the same effect as
(*COMMIT).
The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
It is like (*MARK:NAME) in that the name is remembered for passing back
- to the caller. However, (*SKIP:NAME) searches only for names set with
+ to the caller. However, (*SKIP:NAME) searches only for names set with
(*MARK), ignoring those set by other backtracking verbs.
(*SKIP)
- This verb, when given without a name, is like (*PRUNE), except that if
- the pattern is unanchored, the "bumpalong" advance is not to the next
+ This verb, when given without a name, is like (*PRUNE), except that if
+ the pattern is unanchored, the "bumpalong" advance is not to the next
character, but to the position in the subject where (*SKIP) was encoun-
- tered. (*SKIP) signifies that whatever text was matched leading up to
- it cannot be part of a successful match if there is a later mismatch.
+ tered. (*SKIP) signifies that whatever text was matched leading up to
+ it cannot be part of a successful match if there is a later mismatch.
Consider:
a+(*SKIP)b
- If the subject is "aaaac...", after the first match attempt fails
- (starting at the first character in the string), the starting point
+ If the subject is "aaaac...", after the first match attempt fails
+ (starting at the first character in the string), the starting point
skips on to start the next attempt at "c". Note that a possessive quan-
- tifer does not have the same effect as this example; although it would
- suppress backtracking during the first match attempt, the second
- attempt would start at the second character instead of skipping on to
+ tifer does not have the same effect as this example; although it would
+ suppress backtracking during the first match attempt, the second
+ attempt would start at the second character instead of skipping on to
"c".
- If (*SKIP) is used inside a lookbehind to specify a new starting point
- that is not later than the starting point of the current match, it is
- ignored, and the normal "bumpalong" occurs.
+ If (*SKIP) is used inside a lookbehind to specify a new starting posi-
+ tion that is not later than the starting point of the current match,
+ the position specified by (*SKIP) is ignored, and instead the normal
+ "bumpalong" occurs.
(*SKIP:NAME)
@@ -9432,7 +9447,7 @@
REVISION
- Last updated: 20 June 2019
+ Last updated: 21 June 2019
Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
Modified: code/trunk/doc/pcre2partial.3
===================================================================
--- code/trunk/doc/pcre2partial.3 2019-06-20 17:19:13 UTC (rev 1116)
+++ code/trunk/doc/pcre2partial.3 2019-06-21 16:10:17 UTC (rev 1117)
@@ -1,4 +1,4 @@
-.TH PCRE2PARTIAL 3 "22 December 2014" "PCRE2 10.00"
+.TH PCRE2PARTIAL 3 "21 June 2019" "PCRE2 10.34"
.SH NAME
PCRE2 - Perl-compatible regular expressions
.SH "PARTIAL MATCHING IN PCRE2"
@@ -326,7 +326,13 @@
Partial match: 123ab
<<<
.P
-3. Because a partial match must always contain at least one character, what
+3. The maximum lookbehind count is also important when the result of a partial
+match attempt is "no match". In this case, the maximum lookbehind characters
+from the end of the current segment must be retained at the start of the next
+segment, in case the lookbehind is at the start of the pattern. Matching the
+next segment must then start at the appropriate offset.
+.P
+4. Because a partial match must always contain at least one character, what
might be considered a partial match of an empty string actually gives a "no
match" result. For example:
.sp
@@ -339,7 +345,7 @@
"no match" result should be interpreted as "partial match of an empty string"
when the pattern contains lookbehinds.
.P
-4. Matching a subject string that is split into multiple segments may not
+5. Matching a subject string that is split into multiple segments may not
always produce exactly the same result as matching over one single long string,
especially when PCRE2_PARTIAL_SOFT is used. The section "Partial Matching and
Word Boundaries" above describes an issue that arises if the pattern ends with
@@ -380,7 +386,7 @@
data> gsb\e=ph,dfa,dfa_restart
Partial match: gsb
.sp
-5. Patterns that contain alternatives at the top level which do not all start
+6. Patterns that contain alternatives at the top level which do not all start
with the same pattern item may not work as expected when PCRE2_DFA_RESTART is
used. For example, consider this pattern:
.sp
@@ -429,6 +435,6 @@
.rs
.sp
.nf
-Last updated: 22 December 2014
-Copyright (c) 1997-2014 University of Cambridge.
+Last updated: 21 June 2019
+Copyright (c) 1997-2019 University of Cambridge.
.fi
Modified: code/trunk/doc/pcre2pattern.3
===================================================================
--- code/trunk/doc/pcre2pattern.3 2019-06-20 17:19:13 UTC (rev 1116)
+++ code/trunk/doc/pcre2pattern.3 2019-06-21 16:10:17 UTC (rev 1117)
@@ -1,4 +1,4 @@
-.TH PCRE2PATTERN 3 "20 June 2019" "PCRE2 10.34"
+.TH PCRE2PATTERN 3 "21 June 2019" "PCRE2 10.34"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 REGULAR EXPRESSION DETAILS"
@@ -2022,8 +2022,10 @@
.sp
Earlier versions of Perl and PCRE1 used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
-patterns are now accepted, but if any repetition of the group does in fact
-match no characters, the loop is forcibly broken.
+patterns are now accepted, but whenever an iteration of such a group matches no
+characters, matching moves on to the next item in the pattern instead of
+repeatedly matching an empty string. This does not prevent backtracking into
+any of the iterations if a subsequent item fails to match.
.P
By default, quantifiers are "greedy", that is, they match as much as possible
(up to the maximum number of permitted times), without causing the rest of the
@@ -2378,6 +2380,9 @@
(see below). In this case, the result of matching the assertion determines
which branch of the condition is followed.
.P
+Lookaround assertions are atomic. If an assertion is true, but there is a
+subsequent matching failure, there is no backtracking into the assertion.
+.P
Assertion groups are not capture groups. If an assertion contains capture
groups within it, these are counted for the purposes of numbering the capture
groups in the whole pattern. Within each branch of an assertion, locally
@@ -3559,9 +3564,9 @@
first match attempt, the second attempt would start at the second character
instead of skipping on to "c".
.P
-If (*SKIP) is used inside a lookbehind to specify a new starting point that is
-not later than the starting point of the current match, it is ignored, and the
-normal "bumpalong" occurs.
+If (*SKIP) is used inside a lookbehind to specify a new starting position that
+is not later than the starting point of the current match, the position
+specified by (*SKIP) is ignored, and instead the normal "bumpalong" occurs.
.sp
(*SKIP:NAME)
.sp
@@ -3782,6 +3787,6 @@
.rs
.sp
.nf
-Last updated: 20 June 2019
+Last updated: 21 June 2019
Copyright (c) 1997-2019 University of Cambridge.
.fi
