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Section: 32.4.3 [re.matchflag] Status: New Submitter: Jonathan Wakely Opened: 2021-09-27 Last modified: 2021-10-14
Priority: 3
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Discussion:
The standard doesn't say what it means if match_prev_avail is set. Table [tab:re.matchflag] says:
--first is a valid iterator position. When this flag is set the flags match_not_bol and match_not_bow shall be ignored by the regular expression algorithms (32.10 [re.alg]) and iterators (32.11 [re.iter]).
What difference does it make whether --first is a valid iterator position or not?
What behaviour is changed when it's a valid iterator position? The standard doesn't say anything else about that. When do the regex algorithms care about --first?
Examples like regex_match("xa"+1, regex("^a"), match_prev_avail)
and regex_match("xa"+1, regex("\\ba"), match_prev_avail)
are presumably supposed to inspect the character at --first
to determine if there is a match.
The standard doesn't specify that *--first is ever inspected,
only that it's a valid character
(which is a useless guarantee if nothing looks at it).
[2021-10-14; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution: