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--- Comment #3 from Steve Andrews <steven.s.andrews@???> 2012-01-27 18:41:25 ---
Thanks for the prompt and helpful reply!
-Steve
On Fri, Jan 27, 2012 at 3:44 AM, Philip Hazel <ph10@???> wrote:
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> http://bugs.exim.org/show_bug.cgi?id=1203
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> --- Comment #1 from Philip Hazel <ph10@???> 2012-01-27 11:44:16
> ---
> On Thu, 26 Jan 2012, Steve Andrews wrote:
>
> > (1) I need a library function that can perform pattern matching
> substitutions.
> > It would be declared sort of like this:
> >
> > int matchandsub(pattern1,string1,pattern2,string2);
> >
> > For example, suppose I gave this function pattern1="a.c", string1="abc",
> and
> > pattern2="j.k"; string2 is only for output. I want the function to see
> that
> > yes, string1 is a correct match for pattern1 and that the letter b is the
> > substituted letter. Then, it puts 'b' into pattern2, returning it in
> string2
> > as "jbk". In general, pattern1 and pattern2 would have to have the same
> > expression forms (or, more precisely, pattern2 would need to have a
> subset of
> > the expressions used in pattern1).
>
> Thanks for taking the time to post your wishlist item. However, the
> above description is extremely special-purpose to your requirements; I
> cannot see it being of wide general use.
>
> I have always seen PCRE purely as a matching library, partly because
> there is enough work in maintaining that selection of facilities, and
> partly because, apart from some very simple cases, most applications
> that want to do replacements have some kind of rather special
> requirement, as indeed you do.
>
> Having said that, I suppose all you are really asking for is a variant
> on a general search-and-replace, where the Perl-like usage would be
>
> matchandsub("a(.)c", "abc", "j$1k", output)
>
> Perhaps one day somebody who is keen enough will write another library
> that sits on top of PCRE, and provides a number of different
> general-purpose "search and replace" facilities. But don't hold your
> breath.
>
> There is one item in the Contrib directory of the PCRE FTP site that
> does substitution. It is called pcre_subst.tar.gz but I have no
> knowledge of its internals. It may be somewhere to start if you end up
> writing your own code.
>
> > (2) I need regular expression support for pattern permutation. For
> example,
> > it's standard that the pattern "abc|def" matches to either "abc" or
> "def".
> > However, I want to write the pattern "abc&def" and to have this match to
> either
> > "abcdef" or to "defabc". This would be a significant new addition to
> regular
> > expression matching, but I think would be highly compatible with the
> current
> > design philosophy. It would also make PCRE more useful for
> bioinformatics and
> > other applications.
>
> Introducing a new metacharacter such as & is a big step, and of course
> incompatible with Perl and other regex engines. Again, this requirement
> does strike me as somewhat special-purpose.
>
> You can, in fact, write a pattern using existing syntax that does what
> you want in simple cases:
>
> (abc)(def)|(?2)(?1)
>
> However, subroutine calls such as (?2) are atomic, so if "abc" is some
> complicated subpattern, this is not exactly the same as
>
> (abc)(def)|(def)(abc)
>
> Instead, it is equivalent to
>
> (abc)(def)|(?>def)(?>abc)
>
> Applications that need to use very complicated patterns often use
> programs to write the patterns. For example, you could write a function
> such as
>
> make_permuted_pattern("abc", "def", output)
>
> which returned the pattern "abcdef|defabc". However, if you use
> capturing parentheses in either of the parts, there would be issues
> about numbering (relative numbering could help).
>
> As you have probably gathered by now, I am unlikely to work on either of
> these things myself - though of course I can't predict whether anyone
> else will feel inspired.
>
> Philip
>
>
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