`sed` differs from different implementations (eg GNU vs BSD).

Bash does actually have a fair amount of regex support built in too though it's not as well talked about.

Both points taken.

Mine remains: that shell affords capabilities via utilities. Awk (again, with numerous implementations) also offers regex capabilities, some not offerred by sed, and missing a few.

When writing cross-platform scripting, adhering to common standards rises in importance. This is possible, if occasionally limiting.