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] 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
> 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
) 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:
~<div[^>]*><ul[^>]*>~ │ │ │ └─ 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:
/foo[^\/]+bar/i │ │ │ └──────┼─────┴─ 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
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