On Mon, 14 Sep 2009, Spoon Reloaded wrote:
> In Perl I can match palindromes (strings that read the same forwards
> and backwards), and only palindromes, using the following pattern:
> /^(.?|(.)(?1)\2)$/
> However, this pattern does not work in PCRE. It doesn't seem to match
> any string longer than 1 character.
I now understand why this is different to Perl. In the PCRE
documentation about recursion, it says:
In PCRE (like Python, but unlike Perl), a recursive subpattern call is
always treated as an atomic group. That is, once it has matched some of
the subject string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure.
This is the difference. The way you have written the pattern, it tries
for the single character first. If you write the alternatives the other
way round, it works as you expect:
PCRE version 7.9 2009-04-11
/^((.)(?1)\2|.?)$/
a
0: a
1: a
aba
0: aba
1: aba
2: a
aabaa
0: aabaa
1: aabaa
2: a
pqaabaaqp
0: pqaabaaqp
1: pqaabaaqp
2: p
I think I will use this example to make the point more clearly in the
documentation.
Philip
--
Philip Hazel
