DSL: need to be able to clear regex capture groups

[johnkerl]

Repro in #1399

[johnkerl]

From #1399 by @archetyped:

Is there a way to clear regex capture groups after a match is performed so that strings like "\1" will not be replaced by a regex capture group?

Proof of concept:

echo a=testing | mlr put -q '
val = "Test something here";
search = "(something)"i;
pre_match = "\1";
match = val =~ search;
post_match = "\1";
print "Variable (Pre-Match): " . pre_match;
print "Variable (Post-Match): " . post_match;
'

Output:

Variable (Pre-Match): \1
Variable (Post-Match): something

Ideally there would be a way to set post_match to "\1" just like pre_match (rather than it being populated by the regex capture group). Escaping (e.g. "\\1") does not seem to have any effect.

After a lot of debugging, I found this behavior to be the reason why some text replacements were not working as expected. Interestingly even a negative (!=~) match seems to cause strings such as "\1" to be replaced by capture groups from the regex match e.g.:

if ( val !=~ search ) {
    # Stop processing
    return val;
}
# Strings defined here will be replaced by capture groups from the above regex match.
test = "\1";

The capture groups appear to even persist into function calls when a match is performed in an outer (calling) function, which makes it even harder to track down when it results in unexpected behavior.

[johnkerl]

@archeyped what do you think of #1448?

[johnkerl]

@archetyped the reset logic is documented here:

https://miller.readthedocs.io/en/man #in/reference-main-regular-expressions/#resetting-captures

Please let me know if more is needed and we can re-open -- thanks!

(Also, the strmatch and strmatchx functions as in issue #283 are on PR #1448.)

[archetyped]

@johnkerl strmatch/strmatchx DSL functions look great, as does the scoping for regex capture groups when using =~.

Look forward to testing the updates when released.

Thanks!