Perl 5 added several additional features to standard regular expressions, (...) It would have been difficult to choose new single-keystroke metacharacters or new special sequences beginning with "\" to represent the new features without making Perl's regular expressions confusingly different from standard REs. (...)
The solution chosen by the Perl developers was to use (?...) as the extension syntax. "?" immediately after a parenthesis was a syntax error because the "?" would have nothing to repeat, so this didn't introduce any compatibility problems. The characters immediately after the "?" indicate what extension is being used, so (?=foo) is one thing (a positive lookahead assertion) and (?:foo) is something else (a non-capturing group containing the subexpression foo).
Python adds an extension syntax to Perl's extension syntax. If the first character after the question mark is a "P", you know that it's an extension that's specific to Python. Currently there are two such extensions: (?P<name>...) defines a named group, and (?P=name) is a backreference to a named group. If future versions of Perl 5 add similar features using a different syntax, the re module will be changed to support the new syntax, while preserving the Python-specific syntax for compatibility's sake.
http://www.amk.ca/python/howto/regex...00000000000000