hugo/docs/content/en/functions/_common/regular-expressions.md

When specifying the regular expression, use a raw string literal (backticks) instead of an interpreted string literal (double quotes) to simplify the syntax. With an interpreted string literal you must escape backslashes.

Go's regular expression package implements the RE2 syntax. The RE2 syntax is a subset of that accepted by PCRE, roughly speaking, and with various caveats. Note that the RE2 \C escape sequence is not supported.