[Python-ideas] Adding function checks to regex

Sat Mar 19 04:25:57 CET 2011

Some of those who are relative new to regexes sometimes ask how to write 
a regex which checks that a number is in a range or is a valid date. 
Although this may be possible, it certainly isn't easy.
 From what I've read, Perl has a way of including code in a regex, but I 
don't think that's a good idea
However, it occurs to me that there may be a case for being able to call 
a supplied function to perform such checking.
Borrowing some syntax from Perl, it could look like this:
 def range_check(m):
 return 1 <= int(m.group()) <= 10
 numbers = regex.findall(r"\b\d+\b(*CALL)", text, call=range_check)
The regex module would match as normal until the "(*CALL)", at which 
point it would call the function. If the function returns True, the 
matching continues (and succeeds); if the function returns False, the 
matching backtracks (and fails).
The function would be passed a match object.
An extension, again borrowing the syntax from Perl, could include a tag 
like this:
 numbers = regex.findall(r"\b\d+\b(*CALL:RANGE)", text, 
call=range_check)
The tag would be passed to the function so that it could support 
multiple checks.
Alternatively, a tag could always be passed; if no tag is provided then 
None would be passed instead.
There's also the additional possibility of providing a dict of functions 
instead and using the tag to select the function which should be called.
I'd be interested in your opinions.