That already exists as "?" and was used in their example: /first[-_]?name/i Or to use your example, just checking for underscores and not also dashes: /f

That already exists as "?" and was used in their example: /first[-_]?name/i Or to use your example, just checking for underscores and not also dashes: /first_?name/i Backslash is already used to change special characters like "?" from these meanings into just "use this character without interpreting it" (or the reverse, in some dialects).