Remove the $ and it works. And remember that PCRE != Perl.

> Remove the $ and it works.
Yeah but that changes the meaning. I want to enforce that it matches the entire string, and it doesn't do that.

To give another example without a $, consider:

echo preg_match('/^(|.(?1))x/', 'abx'), "\n";

Again, it works in Perl, but not in PHP.

Nevermind, so I think the reason is that PCRE differs from Perl in that a recursive "sub-"pattern (i.e. ?1, ?2, etc. but not ?0) only matches exactly what was matched before by that subgroup, even if there are other unused possibilities by that group. Is that correct?

But that still wouldn't explain why stuff like balanced parentheses matching /^(\(((?>[^()]+)|(?1))*\))$/ works. I am still waiting for an explanation on my example.

I found a solution:

echo preg_match('/^|(.(?:|(?1)))$/', 'ab'), "\n";

but I still don't understand why the other one doesn't work

oops in that last one I meant

echo preg_match('/^$|^(.(?:|(?1)))$/', 'ab'), "\n";

oops my previous workaround still doens't work

echo preg_match('/^$|^(.(?:|(?1)))$/', 'abc'), "\n";

The only time a PCRE pattern related report like this should be submitted as a PHP bug is if you can prove it works in the command line pcre debugging tool, but it breaks in PHP.  Otherwise you should be reporting stuff like this to the PCRE project.