Why do the PHP preg_* functions require regexp delimiters? [closed]

Question

This is bugging me:

Why preg_match('/pattern/', $haystack) instead of preg_match('pattern', $haystack)? Everything I've seen just states as a fact that they're necessary, and mentions that you can use alternate delimiters.

But, it's a function that defines its own interface outside of the string. It has a flags argument. Adding intra-string syntax seems capricious.

Is it something inherited from pcre that the authors were just not interested in working around? Yet another perverse fact of PHP? Or is there a justification?

Answer 1

The delimiters are for compatibility with Perl. Perl regular expressions use the delimiters, and rely on the end delimiter to signify the start of modifier flags, like i for case insensitivity, for example.

// Match alpha-numeric, case insensitive, multiline
preg_match('/^[a-z0-9]+$/im', $input);

The optional flags argument to preg_match() does not implement the regular expression flags like i that follow the second delimiter. They serve a different function, and indeed PREG_OFFSET_CAPTURE is the only flag available there. This is not to say the regex flags could not have been implemented as another function parameter. They certainly could have, but Perl-compatibility is the goal here.

PHP is not the only language that borrows directly from Perl to implement regular expressions. JavaScript does to some degree, and Ruby even implements Perl's =~ operator for regular expression matches.