pregexp over streams, and other random hackery

This page is a mirrored copy of an article originally posted on the LShift blog; see the archive index here.

Over at eighty-twenty I recount a couple of recent random excursions into various bits and pieces of code.

Besides those developments, I also spent some time on Sunday morning modifying Dorai Sitaram’s pregexp version 20050502 to operate over streams as well as strings, so that I could use it for lexing arbitrary character sources (for instance, with the packrat parser library I’ve been developing).

The basic interface, after the patch, is now either the standard

(pregexp-match <pattern> <string>)

or the new

(pregexp-match <pattern> <stream>)

where streams are created with

(pregexp-make-stream <generatorfunction> <seed>)

I’ve also added a procedure pregexp-match-head, which is like pregexp-match-positions except it only matches at the very beginning of the input string or stream; pregexp-match-head behaves like Python’s re.match, where pregexp-match-positions behaves like Python’s re.search.

I haven’t modified pregexp-split, pregexp-replace, or pregexp-replace*, partly because I have no need for them for my application and partly because I’m not sure what their behaviour should be: should pregexp-replace, when given a stream, answer a stream, or a string? In the case of pregexp-split, since it has to examine the entire input in any case, supporting streams seems unnecessary. (Perhaps I should have included a pregexp-stream->string utility, though.)

The patch against version 20050502 is downloadable here.

Thanks to Dorai for a great library!

Update: I noticed a bug in the first revision of the patch. I’ve updated the links in the article above to point to the new patch.