Asked  2 Years ago    Answers:  5   Viewed   503 times

I have the following error:

Warning: preg_replace(): Unknown modifier ']' in xxx.php on line 38

This is the code on line 38:

<?php echo str_replace("</ul></div>", "", preg_replace("<div[^>]*><ul[^>]*>", "", wp_nav_menu(array('theme_location' => 'nav', 'echo' => false)) )); ?>

How can I fix this problem?



Why the error occurs

In PHP, a regular expression needs to be enclosed within a pair of delimiters. A delimiter can be any non-alphanumeric, non-backslash, non-whitespace character; /, #, ~ are the most commonly used ones. Note that it is also possible to use bracket style delimiters where the opening and closing brackets are the starting and ending delimiter, i.e. <pattern_goes_here>, [pattern_goes_here] etc. are all valid.

The "Unknown modifier X" error usually occurs in the following two cases:

  • When your regular expression is missing delimiters.

  • When you use the delimiter inside the pattern without escaping it.

In this case, the regular expression is <div[^>]*><ul[^>]*>. The regex engine considers everything from < to > as the regex pattern, and everything afterwards as modifiers.

Regex: <div[^>  ]*><ul[^>]*>
       ?     ?  ?          ?
       ???????  ????????????
       pattern    modifiers

] here is an unknown modifier, because it appears after the closing > delimiter. Which is why PHP throws that error.

Depending on the pattern, the unknown modifier complaint might as well have been about *, +, p, / or ) or almost any other letter/symbol. Only imsxeADSUXJu are valid PCRE modifiers.

How to fix it

The fix is easy. Just wrap your regex pattern with any valid delimiters. In this case, you could chose ~ and get the following:

?                   ?
?                   ?? ending delimiter
?????????????????????? starting delimiter

If you're receiving this error despite having used a delimiter, it might be because the pattern itself contains unescaped occurrences of the said delimiter.

Or escape delimiters

/foo[^/]+bar/i would certainly throw an error. So you can escape it using a backslash if it appears anywhere within the regex:

?      ?     ?
??????????????? actual delimiters
       ???????? escaped slash(/) character

This is a tedious job if your regex pattern contains so many occurrences of the delimiter character.

The cleaner way, of course, would be to use a different delimiter altogether. Ideally a character that does not appear anywhere inside the regex pattern, say # - #foo[^/]+bar#i.

More reading:

  • PHP regex delimiters
  • How can I convert ereg expressions to preg in PHP? (missing delimiters)
  • Unknown modifier '/' in …? what is it? (on using preg_quote())
Tuesday, August 2, 2022

I assume $value contains a slash /, which is not escaped by preg_quote:

The special regular expression characters are: . + * ? [ ^ ] $ ( ) { } = ! < > | : -

Pass the delimiter you use to the function:

preg_match_all("/[^s]*".preg_quote($value, '/')."[^s]*/iu", $row_search['content'], $final_matched);
//                                        ---^

or use another delimiter.

Friday, August 12, 2022

To port ereg_replace to preg_replace you need to put the regex between a pair of delimiter

Also your regx is [] is invalid to be used for preg_replace as the is escaping the closing char class ]

The correct port is


Also since the char class has just one char there is no real need of char class you can just say:


Since you are replace just a single char, using regex for this is not recommended. You should be using a simple text replacement using str_replace as:

Thursday, November 24, 2022

You need not use look arounds here. It can be written as


replace with 1:2

Regex Demo


preg_replace ("/("[^";]*);([^"]*")/m", "\1:\2", 'asbas;"asd;";asd;asdadasd;"asd;adsas"' );
=> asbas;"asd:";asd;asdadasd;"asd:adsas"



Just replace the matched ; with :


Thursday, September 22, 2022

Your regex will break if the string $id_base has a / in it as you are using / as the regex delimiter.

To fix this use preg_quote on $id_base as:

if (preg_match('/'. preg_quote($id_base,'/').'-([0-9]+)$/', .....) {
Saturday, October 29, 2022
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