I think the Linux expr command is woefully underused for matching and extraction of substrings using basic regular expressions, so herewith, a quick primer.

Consider a fully-qualified package name with version number and, say, patch level number:

$ P="fubar-1.2.3-42"

Say one wanted to match or extract different components of that string – this variation of the expr command will, first, simply count the number of matching characters based on an anchored match; that is, anchored to the beginning of the string.