Perl was a major popularizer and developer of regular expressions as we know them today.
PHP supports two families of regular expressions: perl-compatible regular expressions which are supplied by the PCRE libary, and the POSIX-extended regular expressions. The perl-compatible regular expression functions are faster and much more popular and POSIX library has been deprecated as of PHP 5.3.0 and will not be included in PHP 6.0.
Python supports regulation expressions through the re module, available in the standard lib.
Python's regular expression support is missing several features. These include:
- Proper scoping for (?flag)
- does not support (?flag:...) syntax
- lookbehinds must be fixed-length
- no possessive quantifiers (atomic groupings are supported)
Python 2.7 is planned to feature an extensive overhaul of the re module. This will or may add several missing oft-grumbled about features from the re module, as well as possibly including rewrites of the underlying engine itself.
These changes are organized on a dedicated launchpad project.