"When working with regular expressions in a shell script the
norm is to use grep or sed or some other external command/program.
Since version 3 of bash (released in 2004) there is another option:
bash's built-in regular expression comparison operator '=~'.
"Bash's regular expression comparison operator takes a string on
the left and an extended regular expression on the right. It
returns 0 (success) if the regular expression matches the string,
otherwise it returns 1 (failure)..."
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