Add Use Regex Pattern Matching With Grep as a unix til

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

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+# 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`](https://github.com/asdf-vm/asdf) using the following command.
+
+```bash
+$ 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`).
+
+```bash
+$ 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`.
+
+[source](https://www.digitalocean.com/community/tutorials/using-grep-regular-expressions-to-search-for-text-patterns-in-linux)