Warning: preg_replace(): Unknown modifier

php, preg-replace, regex, wordpress

Solution

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:

~<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 `#` - `#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 '/' in …? what is it? (on using `preg_quote()`)

Problem

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?

Original source

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