Unknown Modifier '/' in ...? What Is It

Unknown modifier '/' error in PHP

Try using a different delimiter, say #:

preg_replace('#http://|ftp://#', 'https://', $value);

or (less recommended) escape every occurrence of the delimiter in the regex:

preg_replace('/http:\/\/|ftp:\/\//', 'https://', $value);

Also you are searching for the pattern http:///ftp:// which really does not make much sense, may be you meant http://|ftp://.

You can make your regex shorter as:

preg_replace('#(?:http|ftp)#', 'https', $value);

Understanding the error: Unknown modifier '/'

In your regex '/http:///ftp:///', the first / is considered as starting delimiter and the / after the : is considered as the ending delimiter. Now we know we can provide modifier to the regex to alter its default behavior. Some such modifiers are:

i : to make the matching case
insensitive
m : multi-line searching

But what PHP sees after the closing delimiter is another / and tries to interpret it as a modifier but fails, resulting in the error.

preg_replace returns the altered string.

$value = 'http://foo.com';
$value = preg_replace('#http://|ftp://#', 'https://', $value);
// $value is now https://foo.com

PHP | preg_match(): Unknown modifier '(' in C:\xampp\htdocs\Folder\index.php on line 38

You forgot one "\" before "/)" on your pattern :

$ptn = "/^http:\/\/steamcommunity\.com\/openid\/id\/(7[0-9]{15,25}+)$/";

So php is thinking that "/(" is a modifier : http://php.net/manual/sr/reference.pcre.pattern.modifiers.php

Warning: preg_match(): Unknown modifier ''

preg_match arguments are in the wrong order. First should be the regular expression.

So change:

preg_match($source_html, $rule, $matches);

to:

preg_match($rule, $source_html, $matches);

preg_match error Unknown modifier '('

Escape / in the regular expression.

"/^([1-9]|0[1-9]|[1,2][0-9]|3[0,1])\/([1-9]|1[0,1,2])\/\d{4} ([0-1][0-9]|[2][0-3])(:([0-5][0-9])){1,2}(:([0-5][0-9])){1,2}$/"
#                                   ^                 ^

Otherwise, / is recognised as end of the regular expression.

Or use different delimiters:

"#^([1-9]|0[1-9]|[1,2][0-9]|3[0,1])/([1-9]|1[0,1,2])/\d{4} ([0-1][0-9]|[2][0-3])(:([0-5][0-9])){1,2}(:([0-5][0-9])){1,2}$#"

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:

~<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.

preg_match(); - Unknown modifier '+'

You need to use delimiters with regexes in PHP. You can use the often used /, but PHP lets you use any matching characters, so @ and # are popular.

preg_replace(): Unknown modifier '\\'

preg_replace(): Unknown modifier occurs when $test contains a /.

preg_quote( $test );

should be

preg_quote( $test, '/' );

/ is the PCRE delimiter used in your '/(^|\W)...(\W|$)/i' expression.

PHP PCRE's can have any delimiter, so you have to tell preg_quote() which delimiter is used.

http://php.net/manual/en/regexp.reference.delimiters.php