`\?`, `\+`, `\|`, `\{m\}`, `\{m,\}` and `\{m,n\}` in BRE

[e-kwsm]

revision: 3504109

From POSIX BRE:

The '?', '+', and '|' characters; it is implementation-defined whether "?", "+", and "|" each match the literal character '?', '+', or '|', respectively, or behave as described for the ERE special characters '?', '+', and '|', respectively (see 9.4.3 ERE Special Characters).

Note:

A future version of this standard may require "\?", "\+", and "\|" to behave as described for the ERE special characters '?', '+', and '|', respectively.

GNU and BSD sed’s treat \?, \+, and \| in BRE as the ERE counterparts, whereas uutils does not:

$ cargo run --quiet -- -n -e '/cats\?$/p' <<< $'cat\ncats?'
cats?
$ sed -n -e '/cats\?$/p' <<< $'cat\ncats?'
cat
$ cargo run --quiet -- -n -e '/se\+d/p' <<< $'seed\nse+d'
se+d
$ sed -n -e '/se\+d/p' <<< $'seed\nse+d'
seed
$ cargo run --quiet -- -n -e '/foo\|bar/p' <<< $'foo\nfoo|bar'
foo|bar
$ sed -n -e '/foo\|bar/p' <<< $'foo\nfoo|bar'
foo
foo|bar

POSIX does not mandate the behavior, but it would be preferable.

[e-kwsm]

When a BRE matching a single character, a subexpression, or a back-reference is followed by an interval expression of the format "\{m\}", "\{m,\}", or "\{m,n\}", together with that interval expression it shall match what repeated consecutive occurrences of the BRE would match. The values of m and n are decimal integers in the range 0<=m<=n<={RE_DUP_MAX}, where m specifies the exact or minimum number of occurrences and n specifies the maximum number of occurrences. The expression "\{m\}" shall match exactly m occurrences of the preceding BRE, "\{m,\}" shall match at least m occurrences, and "\{m,n\}" shall match any number of occurrences between m and n, inclusive.

The brace quantifiers are mandatory but currently uutils does not support them.

[LoukasPap]

I'll check that out too.