Merge branch 'rs/use-enhanced-bre-on-macos'

Newer regex library macOS stopped enabling GNU-like enhanced BRE, where '\(A\|B\)' works as alternation, unless explicitly asked with the REG_ENHANCED flag. "git grep" now can be compiled to do so, to retain the old behaviour. * rs/use-enhanced-bre-on-macos: use enhanced basic regular expressions on macOS