til/unix/use-regex-pattern-matching-with-grep.md

Use Regex Pattern Matching With Grep

The grep command supports perl-flavored regular expression pattern matching. Rather than grepping for specific words, you can use regex with grep to find patterns throughout a text or command output.

As an example, I can list all Ruby versions available for install with asdf using the following command.

$ asdf list-all ruby

This produces a ton of lines of output including versions of jruby and truffleruby.

I can use grep to filter this list down to the MRI versions which all start with a digit (e.g. 2.6.5).

$ asdf list-all ruby | grep "^[[:digit:]]"

That regex says, find all lines that begin (^) with a number ([[:digit:]]). This means grep will filter down the output to things like 1.9.3-p551, 2.6.5, and 2.7.0-preview2 whereas it will exclude truffleruby-19.0.0 and jruby-9.2.9.0.

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