content: Consolidate configuration documentation

content/en/quick-reference/glossary/regular-expression.md

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+---
+title: regular expression
+reference: 
+---
+
+A _regular expression_, also known as a _regex_, is a sequence of characters that defines a search pattern. Use the [RE2 syntax] when defining regular expressions in your templates or site configuration.
+
+  [RE2 syntax]: https://github.com/google/re2/wiki/syntax