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### 3605. `regex_constants::match_prev_avail` is underspecified

Section: 30.4.3 [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 (30.10 [re.alg]) and iterators (30.11 [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: