better way to get last named capture group

ysth has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: better way to get last named capture group by NERDVANA (Priest) on Jul 01, 2025 at 16:37 UTC
I don't believe that specific feature exists, but perl has a lot of other quirky tools in the regex engine, and you haven't really explained what real problem you're trying to solve. At a glance, I can't think how knowing "lastgroup" would be useful in real-world problems, except in the case where you were trying to discover which of N capture groups were found. If the goal was to perform a more efficient "switch" to execute code after specific captures were detected, the perl tool for that is to embed perl code directly into the regex so that it runs right after the capture. `perl -E 'say "xxx" =~ s/^.(?{"a"})\|.$(?{"c"})\|.(?{"b"})/$^R/gr'` [download] or with /x `say "xxx" =~ s/ ^. (?{ "a" }) \| .$ (?{ "c" }) \| . (?{ "b" }) /$^R/grx'` [download] The advantage here is that you don't have to re-dispatch based on which matched; you are immediately running code based on the pattern. The disadvantage is that you have to pay close attention to how this feature interacts with backtracking, because the perl regex engine may run your code block before deciding that the pattern doesn't match, and then run different code for the same characters. In this case, I put all the code to execute after the pattern has been matched. In general, try to avoid side effects and return a value (to $^R) that can be used only once the replacement is determined to have succeeded. If you need side effects, you can also make use of the various regex directives that prevent backtracking. (see `perldoc re` and search for "backtrack") I can dig up examples of how you can expand this to a full language parser, if you're interested.	[reply] [d/l] [select]
Re^2: better way to get last named capture group by ysth (Canon) on Jul 01, 2025 at 20:32 UTC
I was trying to stick to non perl specific regex features. There's `(*MARK:NAME)` as a similar alternative, not sure what all supports that besides perl and pcre though. Edit: according to https://rbuckton.github.io/regexp-features/features/backtracking-control-verbs.html, nothing else supports it.	[reply] [d/l]
Re: better way to get last named capture group by ikegami (Patriarch) on Jul 01, 2025 at 15:28 UTC
`%-` and `%+` are the only var that contains capture names.	[reply] [d/l] [select]
Re: better way to get last named capture group by ysth (Canon) on Jul 01, 2025 at 22:57 UTC
An alternative that feels cleaner, though `(MARK:NAME)` is supported way fewer places than named captures: `$ perl -E'say "xxx" =~ s/(MARK:a)^.\|(MARK:c).$\|(MARK:b)./$REGMARK/g +r' abc` [download] `$ python -c 'import pcre2; print(pcre2.sub("(MARK:a)^.\|(MARK:c).$\|(* +MARK:b).", "$*MARK", "xxx"))' abc` [download]	[reply] [d/l] [select]
Re: better way to get last named capture group by sleet (Monk) on Jul 01, 2025 at 18:31 UTC
For your example, wouldn't a branch reset be more appropriate?	[reply]
Re^2: better way to get last named capture group by ysth (Canon) on Jul 01, 2025 at 20:21 UTC
Exactly the opposite! I want to distinguish which branch matched, when the matched strings could all be the same.	[reply]
Re^3: better way to get last named capture group by ikegami (Patriarch) on Jul 01, 2025 at 22:02 UTC
With numbered captures, `defined( $1 )` tells us if a capture is part of the matching patch. The equivalent with named captures would be `exists( $+{ name } )`. To check which of a set, `( grep exists( $+{ $_ } ), qw( name1 name2 name3 ) )[ 0 ]` could be used. And of course, that simplifies to `( keys( %+ ) )[ 0 ]` if only one named capture could have captured.	[reply] [d/l] [select]
Re: better way to get last named capture group by LanX (Saint) on Jul 02, 2025 at 23:21 UTC
If your goal was to mimic the exact Python semantic, I'd try to patch or subclass Tie::Hash::NamedCapture to record the last name set. > Internally %+ and %- are implemented with a real tied interface via Tie::Hash::NamedCapture In a hurry I couldn't find the implementation, not sure if it was done in pure Perl or XS/C. But even if in C, monkey patching STORE with a wrapper should do the job update Never mind. I ran some experiments and couldn't succeed in patching STORE to any effect. And according to the docs > %- and %+ are tied views into a common internal hash associated with the last successful regular expression. this wouldn't help either, because the internal hash is out of reach ... too bad. Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply]
Re^2: better way to get last named capture group by ysth (Canon) on Jul 03, 2025 at 02:25 UTC
The source is in perl's universal.c, but the update tie methods don't do anything; the read tie methods call named_buff or named_buff_iter to get the data from the regex engine. So no patching possible.	[reply]
Re^3: better way to get last named capture group by LanX (Saint) on Jul 04, 2025 at 10:53 UTC
Yeah, the tie is basically only a frontend to the regex engine. One side effect is that you can tie other %hashes to act like %+ and %- Cheers Rolf _{(addicted to the Perl Programming Language :) see Wikisyntax for the Monastery}	[reply]
Re^2: better way to get last named capture group by ikegami (Patriarch) on Jul 03, 2025 at 10:56 UTC
It's in universal.c. It would be possible to modify Perl to provide something like `tied( %+ )->latest_capture_name` [download]	[reply] [d/l]

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