Correct way to add comments to a Regular Expression in PHP

Question

I'm trying to add comments to make a regexp clearer

// Strip any URLs (such as embeds) taken from http://stackoverflow.com/questions/6427530/regular-expression-pattern-to-match-url-with-or-without-http-www
$pattern =
    '(                               # First capturing group
            (http|https)             # Second capturing grout,matches wither http or https
        \:\/\/)?                     # End of first capturing group, matches :// exactly
        [                            # Match any char in the following list. the + after the closing bracke means greedy
            a-z                      # Any char between a and z
            A-Z                      # Any char between A and Z
            0-9                      # Any char between 0 and 9
            \.\/\?\:@\-              # ./?:@- literally ( any one of them )
            _=#                      # _=# any of these thre chars
        ]+                           # end of list
        \.                           # matches .
        (                            # third caturing group
            [                        # start of list
                a-z                  # Any char between a and z
                A-Z                  # Any char between A and Z
                0-9                  # Any char between 0 and 9
                \.\/\?\:@\-          # ./?:@- literally ( any one of them )
                _=#                  # _=# any of these thre chars
            ]                        # end of list
        )*                           # end of capturing group with greedy modifier';
$excerpt = preg_replace("/$pattern/x", '', $excerpt );

But i get the warning

Warning: preg_replace(): Unknown modifier '/' in on line 280

How should i comment it?

Answer 1

This may not be the cleanest approach, but you could enclose each section in quotes and concatenate them.

Something like this should work:

$pattern =
    '('.                             // First capturing group
        '(http|https)'.              // Second capturing grout,matches wither http or https
    '\:\/\/)?'.                      // End of first capturing group, matches :// exactly
    ...   

---

Alternatively I found this in PHP docs.

So I imagine that would work too, but you are using the x modifier and that should be working already.

If the PCRE_EXTENDED option is set, an unescaped # character outside a character class introduces a comment that continues up to the next newline character in the pattern.

This indicates all of you comments within a character set [...] are invalid.

Here is a working example for use with the PCRE_EXTENDED modifier:

$pattern = '
    (                              # First capturing group
        (http[s]?)                 # Second capturing grout,matches wither http or https
    \:\/\/)?                       # End of first capturing group, matches :// exactly
    [a-zA-Z0-9\.\/\?\:@\-_=#]+     # [List Comment Here]
    \.                             # matches .
    (                              # third caturing group
        [a-zA-Z0-9\.\/\?\:@\-_=#]  # [List Comment Here]
    )*                             # end of capturing group with greedy modifier
';

Answer 2

This was brought up in a comment on the php.net modifiers page.

To quote:

When adding comments with the /x modifier, don't use the pattern delimiter in the comments. It may not be ignored in the comments area.

In your example, one of your comments has the string :// embedded within it. Since PHP seems to not parse regex delimiters by taking into account flags, it sees this as a problem. The same can be seen with the below code:

echo preg_replace('/
a #Com/ment
/x', 'e', 'and');

Demo

You would need to either change your delimiter or escape the delimiter in comments.