Warning: preg_match() [function.preg-match]: Unknown modifier '-'

Question

I am working on a Wordpress site and recently I have begun getting this warning:

Warning: preg_match() [function.preg-match]: Unknown modifier '-'

It started when I changed the permalink structure to /%postname%/, which is needed for BuddyPress to function. If use the default permalink structure, the problem goes away.

Here is the code from the wp-includes/class-wp.php where the error is occurring:

if ( preg_match("#^$match#", $request_match, $matches) ||
 preg_match("#^$match#", urldecode($request_match), $matches) ) {

Possible duplicate of [Warning: preg\_replace(): Unknown modifier '\]'](https://stackoverflow.com/questions/20705399/warning-preg-replace-unknown-modifier) — Toto, Jul 04 '19 at 13:38

Iłya Bursov · Answer 1 · 2013-10-24T22:27:58.710

1

this is because - and / are special symbols, you can change this code to:

if ( preg_match("/^".preg_quote($match)."/", $request_match, $matches) ||
 preg_match("/^".preg_quote($match)."/", urldecode($request_match), $matches) ) {

but I assume that problem is somewhere deeper, in core logic of wp

edited Oct 24 '13 at 22:27

answered Oct 24 '13 at 21:41

Iłya Bursov

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Thanks Ilya, but the issue remains. You are most likely correct that the problem lies somewhere deeper. – John Smiley Oct 24 '13 at 21:55
issue in the same lines? – Iłya Bursov Oct 24 '13 at 22:10
`-` doesn't have a special meaning outside a character class. – Casimir et Hippolyte Oct 24 '13 at 22:25
@CasimiretHippolyte yes, value has `/` too, so `-` became modifier – Iłya Bursov Oct 24 '13 at 22:28

score 0 · Answer 2 · answered Oct 24 '13 at 22:31

Without the $match variable content it is difficult to know what the problem is, but if you obtain this warning, it's because $match contains #- (i.e. the pattern delimiter used and the - character). Then all characters after this # are seen as modifiers.

You can try to change the delimiter (and pray) to ~:

if ( preg_match("~^$match~", $request_match, $matches) ||
preg_match("~^$match~", urldecode($request_match), $matches) ) {

If it doesn't work try other delimiters.

Warning: preg_match() [function.preg-match]: Unknown modifier '-'

2 Answers2