use preg_replace to replace character unless an escape character precedes

Question

i'm trying to do the following hope there's a reg_ex expert around to shed some light. I need to replace the character [ in my code and make it a {. But there is cases where the [ needs to remain a [ and not change. So the way i figured it is i need to use the

preg_replace("[", "{", $string);

function with a suitable regular expression that will result the [ characters that are not preceded by the escape character to be used lets say . So how can i get to replace this "[" and not this "["?

thanks a lot in advance

Answer 1

You can do this, and it's with a convention call negative lookbehind assertions

echo preg_replace( "/(?<!\\\\)\\[/", '{', "\[ [ [ \[ [" );

Basically, "replace [ with { only when not preceded by \"

Answer 2

For completeness:

There another way to do this if your regexp engine is primitive and doesn't know how to do negative lookbehinds (which is not the case with PHP):

$str = preg_replace( "/(^|[^\\\\])\\[/", '$1{', $str );

This captures the preceding character, which is restricted to ^ (start) or [^\] (any non-slash), and passes it through the replace.