regex(3): Add test to cover recent BRE regression

[kevans]

Summary

BREs recently became prematurely sensitive to the branching
operator, which outright broke expressions that used it instead of
failing silently. Test that \| is matching a literal | for the time
being.

[cem]

Are \(\) not matching literal parentheses? I guess I'm confused why the 4th column is b|c (and why the first is b). (Also, I'm unfamiliar with this input format and just guessing at what things mean.)

[kevans]

In D11577#240230, @cem wrote:

Are \(\) not matching literal parentheses? I guess I'm confused why the 4th column is b|c (and why the first is b). (Also, I'm unfamiliar with this input format and just guessing at what things mean.)

Correct- In BRE's, \( and \) specify a matching group as () do in EREs.

The first column 'b' means "try this as a BRE only", the second column is the string to try matching it against, the third is the section of the string that the expression matches (all of it, in this case), and the fourth is a comma-separated list of what the subexpressions (matching groups) should match

[cem]

Got it! Makes sense. (And consider it approved for commit, if Ed's still out.)

[kevans]

Ping @emaste