[Python-Dev] Adding function checks to regex

[Guido van Rossum]

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 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.
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-- 
--Guido van Rossum (python.org/~guido)