empty character class [] matches empty string

[skvadrik]

E.g. the following source:

/*!re2c
    [] {}
*/

compiles to the following code:

{
        YYCTYPE yych;

        {}
}

Reproducible with 0.13.6, 0.14 and HEAD (and seems that it has always been that way). It of course should never match (and preferably report a warning).

Original comment by: skvadrik

[skvadrik]

I'm thinking of completely forbidding empty character classes. It seems to conform perl and posix BRE/ERE regular expressions.

Original comment by: skvadrik

[skvadrik]

Further analyses of the following case:

/*!re2c
    [] {}
*/

/*!re2c
    [^\x00-\xFF] {}
*/

/*!re2c
    [\x00-\xFF]\[\x00-\xFF] {}
*/

showed that versions <=0.13.6 and >=0.13.7 behave differently.

Up to 0.13.6 re2c consistently considered that empty range should match empty string:

$ re2c -i --no-generation-date 1.re 
/* Generated by re2c 0.13.6 */

{
        YYCTYPE yych;

        {}
}



{
        YYCTYPE yych;
        {}
}



{
        YYCTYPE yych;
        {}
}

Starting from 0.13.7 this behaviour was broken (the faulty commit is unsurprisingly the big one that added UTF8 support). empty positive range [] and empty difference (e.g. [a-z][a-z]) still match empty string, but empty negative range (e.g. [^\x00-\xFF]) matches nothing (always fails):

$ re2c -i --no-generation-date 1.re 
/* Generated by re2c 0.14.1.dev */

{
        YYCTYPE yych;

        {}
}



{
        YYCTYPE yych;
}



{
        YYCTYPE yych;
        {}
}

Whether we choose to match empty string or match nothing on empty ranges, the behaviour must be consistent (apply to all cases of range construction).

Original comment by: skvadrik

[skvadrik]

I vote for making it an error (or at least invalid according to the re2c grammar). As far as my imagination goes, it's a pointless thing to do.

Looking at the posix standard for regexes, they're not allowed: "A bracket expression (an expression enclosed in square brackets, "[]" ) is an RE that shall match a single collating element contained in the non-empty set of collating elements represented by the bracket expression." http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html

Original comment by: nuffer

[skvadrik]

I agree entirely, but you know, re2c users are quite conservative ;)

I think I'll add an option "--empty-class <match-empty|match-none|error>" and default to "match-empty" (match empty input, as in <=0.13.6) in 0.15, then switch the default to "error" in 0.16 (if nobody objects to it).

Anyway, a warning will be useful.

Original comment by: skvadrik

[skvadrik]

This commit adds the "--empty-class" option.

Dan, does it look good to you?

Original comment by: skvadrik

[skvadrik]

+1 Looks good.

Original comment by: nuffer

[skvadrik]

• status: accepted --> closed-fixed

Original comment by: skvadrik