test for regex in string with a posix shell

how can I test if a string matches a particular string matches regex with a basic (no bash or anything) posix shell script (in an if statement)?

You can use expr command to evaluate regular expression in a POSIX shell:

s='Abc'
expr $s : '^[[:alpha:]]\+'

3

• expr returns # of matched characters which is 3 in this case.

• Regular expression ^[[:alpha:]]\+ does the following:

  • ^: Match start positon

  • [[:alpha:]]\+: Match 1 or more of any ASCII alphabets