[Python-Dev] Adding function checks to regex (original) (raw)

Guido van Rossum guido at python.org
Sat Mar 19 01:38:33 CET 2011

Probably best discussed on python-ideas...

On Fri, Mar 18, 2011 at 4:04 PM, MRAB <python at mrabarnett.plus.com> wrote:

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 rangecheck(m):  return 1 <= int(m.group()) <= 10  numbers = regex.findall(r"\b\d+\b(*CALL)", text, call=rangecheck) 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=rangecheck) 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.

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