CODEWITHSUNDEEP
phpregex
Dail
Dail

Reputation: 4608

What are those characters in a regular expression?

I found this regex that works correctly but I didn't understand what is # (at the start) and at the end of the expression. Are not ^ and $ the start/end characters?

preg_match_all('#^/([^/]+)/([^/]+)/$#', $s, $matches);

Thanks

Upvotes: 1

Views: 101

Answers (6)

Benjie
Benjie

Reputation: 7946

Let me break it down:

# is the first character, so this is the character used as the delimiter of the regular expression - we know we've got to the end when we reach the next (unescaped) one of these

^ outside of a character class, this means the beginning of the string

/ is just a normal 'slash' character

([^/]+) This is a bracketed expression containing at least one (+) instance of any character that isn't a / (^ at the beginning of a character class inverts the character class - meaning it will only match characters that are not in this list)

/ again

([^/]+) again

/ again

$ this matches the end of the string

# this is the final delimeter, so we know that the regex is now finished.

Upvotes: 0

Daniel Böhmer
Daniel Böhmer

Reputation: 15411

The matched pattern contains many /, thus the # is used as regex delimeter. These are identical

/^something$/

and 

#^something$#

If you have multiple / in your pattern the 2nd example is better suited to avoid ugly masking with \/. This is how the RE would like like with using the standard // syntax:

/^\/([^\/]+)\/([^\/]+)\/$/

Upvotes: 3

Niet the Dark Absol
Niet the Dark Absol

Reputation: 324750

You can use pretty much anything as delimiters. The most common one is /.../, but if the pattern itself contains / and you don't want to escape any and all occurrences, you can use a different delimiter. My personal preference is (...) because it reminds me that $0 of the result is the entire pattern. But you can do anything, <...>, #...#, %...%, {...}... well, almost anything. I don't know exactly what the requirements are, but I think it's "any non-alphanumeric character".

Upvotes: 0

Luke
Luke

Reputation: 23690

These are delimiters. You can use any delimiter you want, but they must appear at the start and end of the regular expression.

Please see this documentation for a detail insight in to regular expressions: http://www.php.net/manual/en/pcre.pattern.php

Upvotes: 0

hakre
hakre

Reputation: 198119

About #:

That's a delimiter of the regular expression itself. It's only meaning is to tell which delimiter is used for the expression. Commonly / is used, but others are possible. PCRE expressions need a delimiter with preg_match or preg_match_all.

About ^:

Inside character classes ([...]), the ^ has the meaning of not if it's the first character.

[abc]   :  matching a, b or c
[^abc]  :  NOT matching a, b or c, match every other character instead

Upvotes: 1

FailedDev
FailedDev

Reputation: 26930

Also # at the start and the end here are custom regex delimiters. Instead of the usual /.../ you have #...#. Just like perl.

Upvotes: 0

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