grimace: a fluent regular expression generator in Python

Ben Last ben at benlast.com
Wed Jul 17 04:33:17 CEST 2013


On 16 July 2013 20:48, <python-list-request at python.org> wrote:

> From: "Anders J. Munch" <2013 at jmunch.dk>
> Date: Tue, 16 Jul 2013 13:38:35 +0200
> Ben Last wrote:
>
>> north_american_number_re = (RE().start
>> .literal('(').followed_by.**exactly(3).digits.then.**literal(')')
>>                                      .then.one.literal("-").then.**
>> exactly(3).digits
>> .then.one.dash.followed_by.**exactly(4).digits.then.end
>>                                      .as_string())
>>
>
> Very cool.  It's a bit verbose for my taste, and I'm not sure how well it
> will cope with nested structure.
>

I guess verbosity is the aim, in that *explicit is better than implicit* :)
 And I suppose that's one of the attributes of a fluent system; they tend
to need more typing.  It's not Perl...



> The problem with Perl-style regexp notation isn't so much that it's terse
> - it's that the syntax is irregular (sic) and doesn't follow modern
> principles for lexical structure in computer languages.  You can get a long
> way just by ignoring whitespace, putting literals in quotes and allowing
> embedded comments.
>

Good points.  I wanted to find a syntax that allows comments as well as
being fluent:

RE()
.any_number_of.digits  # Recall that any_number_of includes zero
.followed_by.an_optional.dot.then.at_least_one.digit  # The dot is
specifically optional
# but we must have one digit as a minimum
.as_string()

... and yes, I aso specifically wanted to have literals quoted.

Nested groups work, but I haven't tackled lookahead and backreferences :
essentially because if you're writing an RE that complex, you should
probably be working directly in RE strings.

Depending on what you mean by "nested", re-use of RE objects is easy
(example from the unit tests):

identifier_start_chars = RE().regex("[a-zA-Z_]")
identifier_chars = RE().regex("[a-zA-Z0-9_]")

self.assertEqual(RE().one_or_more.of(identifier_start_chars)
                     .followed_by.zero_or_more(identifier_chars)
                     .as_string(),
                     r"[a-zA-Z_]+[a-zA-Z0-9_]*")


Thanks for the comments!
ben
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