On Mon, Feb 11, 2019 at 7:08 PM Philip McGrath <phi...@philipmcgrath.com> wrote:
>
> On Mon, Feb 11, 2019 at 2:32 PM David Storrs <david.sto...@gmail.com> wrote:
>>
>> It would be great if Perl's / pcre's "x" mode could be pulled into
>> Racket, ideally with modifiers on the end of the regexp instead of
>> inside it. This mode specifies that all whitespace in the pattern
>> should be ignored, so if you want to actually match whitespace then
>> you need to specify it. Example:
>>
>> ; tokenizer code running under the at-exp sublanguage: match a word
>> boundary, 'var', whitespace, one or more contiguous word characters
>> (ASCII alphanumerics or _), and then another word boundary
>>
>> @pregexp{\b var \s+ \w+ \b}x
>
>
> Would this do what you want?
>
> #lang at-exp racket
>
> (define (pregexp-x . args)
> (pregexp
> (regexp-replace* #px"\\s+" (string-append* args) "")))
>
> @pregexp-x{\b var \s+ \w+ \b} ;; -> #px"\\bvar\\s+\\w+\\b"
>
> (With an easy optimization being to make `pregexp-x` a macro so that it can
> do that work at compile-time when all of the args are string literals.)
>
It would, yes. Thanks. Although, see below.
>> Oh, and if I'm already asking for the moon, does Racket have any
>> equivalent to Perl's named captures or composable regexen, or any
>> possibility of adding them?
>>
>> (regexp-match @px{\b(?<greet>hi|hello)} "hi bob")
>> (println greet) ; prints "hi"
>>
>
> I think both of these could be implemented well in "user space."
> [code]
That's impressive and yes, you're right that it works. The problem is
that it is something non-standard, a user extension that the
programmer needs to know about, ensure is installed, and require.
Perl, Python, and the pcre library all consider named captures to be
first-class features of the language. Perl considers regex
composition to be first-class, and Python and pcre probably do as well
although I haven't checked. If you end up doing a lot of text
handling then these features are invaluable, so why is Racket behind
the times?
Actually, that raises a question: Does Racket use the pcre library or
does it reinvent the wheel? If it uses pcre then why not make all the
features available?
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