Regular expression: Unknown modifier '\'

Question

I'm having the following regular expression:

(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'\".,<>?«»“”‘’]))

but it's giving me the following error:

Unknown modifier '\'

I tried escaping the backslashes like this:

(?i)\\b((?:[a-z][\\w-]+:(?:/{1,3}|[a-z0-9%])|www\\d{0,3}[.]|[a-z0-9.\\-]+[.][a-z]{2,4}/)(?:[^\\s()<>]+|\\(([^\\s()<>]+|(\\([^\\s()<>]+\\)))*\\))+(?:\\(([^\\s()<>]+|(\\([^\\s()<>]+\\)))*\\)|[^\\s`!()\\[\\]{};:'\".,<>?«»“”‘’]))

Without any luck.. I also tried replacing the backslashes with tildes, again without luck. I've searched the internet and SO for any details about the '\' as unknown modifier but again without luck. What is going wrong here?

A code sample included on request:

$regex = '(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:\'\".,<>?«»“”‘’]))';
return !preg_match($regex, $url);

Thanks!

Backslash is used for escaping special characters. You have to escape the whole regex to use it properly, Shankar Damodaran already showed how. — Realitätsverlust, Dec 11 '13 at 08:57
How do you use this regex? Show us some code, I suspect your're not using delimiter. — Toto, Dec 11 '13 at 09:01
Possible duplicate of [Warning: preg\_replace(): Unknown modifier '\]'](https://stackoverflow.com/questions/20705399/warning-preg-replace-unknown-modifier) — Munim Munna, Jul 06 '19 at 07:28

score 7 · Answer 1 · edited May 23 '17 at 12:00

Just escape the single quote using a backslash like this

$regex = '(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:\'\".,<>?«»“”‘’]))';

EDIT :

Regarding Unknown Modifier Warning..

I assume that you may have got that on your preg functions.

A preg pattern needs a pair of characters which delimit the pattern itself.

You should add a delimiter sort of thing see here...

$body = preg_replace("/(.*)<!-- start -->(.*)<!-- end -->(.*)/","$2",$body);
                ------^                                 -----^

Source

This actually doesn't seem to solve the issue. It's still throwing an unknown modifier "\" warning. — Fabian Pas, Dec 11 '13 at 09:03

Toto · Accepted Answer · 2013-12-11T09:20:57.813

5

You have to add delimiter arround your regex:

$regex = '~(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:\'\".,<>?«»“”‘’]))~';

As delimiter, you can use almost any character except spaces or word character.
I recommend you to use a character that is not present in your regex, so you haven't to escape it.
Here I'm using ~

edited Dec 11 '13 at 09:20

answered Dec 11 '13 at 09:06

Toto

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This did the job! I understand it now. Regular expressions seem like a puzzle to me sometimes.. – Fabian Pas Dec 11 '13 at 09:23
1

@FabianPas, True. That's why i never venture too far on that. – Shankar Narayana Damodaran Dec 11 '13 at 09:30
I wasn't aware you could define your own delimiter like this... – Jacob Thomason Dec 07 '20 at 10:41
1

@JacobThomason: You can use any non-alphanumeric, non-backslash, non-whitespace character [as delimiter](https://www.php.net/manual/en/regexp.reference.delimiters.php) – Toto Dec 07 '20 at 11:17

score 3 · Answer 3 · answered Dec 11 '13 at 11:22

Since I am missing an answer that explains the warning, I add another one.

The preg functions need a regex delimiter and you forgot to put one explicitly, so the first character in the regex string is used as delimiter. Since it is an opening bracket, the according delimiter is the closing bracket.

$regex = '(?i)\b((?:[a-z][\w-]+:(?:/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:\'\".,<>?«»“”‘’]))';
          |  || 
          |  || first character after the closing delimiter is an unknown modifier ==> your warning !
          |  | first closing bracket is the other delimiter
          | first opening bracket is the delimiter

That is because as regex delimiters also a pair of brackets can be used. See php doc on Delimiters

So the solution is to put an appropriate regex delimiter, like the other people here showed you.

Regular expression: Unknown modifier '\'

3 Answers3