Warning: preg_replace(): Unknown modifier

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
  • http://www.regular-expressions.info/php.html
  • How can I convert ereg expressions to preg in PHP? (missing delimiters)
  • Unknown modifier “https://stackoverflow.com/” in …? what is it? (on using preg_quote())

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