regular expressions php/perl/actionscript

Hello I'm wondering if Regular Expressions are the same in Perl and PHP (and possibly Actionscript)? They look the same and smell the same, but if I see a book in a store for Perl Regular Expressions that's $10 cheaper than the PHP one (is there one?), then is it the same thing? While we're on this topic, I'm looking to find an attribute in an XML and replace it with a string variable I get from a form. I can use: $bgClrOld = preg_match('/backColour/',$s); // 1 to find the attribute, but how do I find and replace backColour="a3b4c5" with backColour=$bgClrNew ? TIA Gabino