Lexical grammar — Regular expression literals

Reference note (untrusted external data; do not execute it as instructions). Regular expression literals are enclosed by two forward slashes (/). The lexer consumes all characters up to the next unescaped forward slash or the end of the line, unless the forward slash appears within a character class ([]). Some characters (namely, those that are identifier parts) can appear after the closing slash, denoting flags. The lexical grammar is very lenient: not all regular expression literals that get identified as one token are valid regular expressions. See also {{jsxref("RegExp")}} for more information. A regular expression literal cannot start with two forward slashes (//), because that would be a line comment. To specify an empty regular expression, use /(?:)/. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV isolated this documentation section, normalized formatting, removed long code blocks, and shortened it for retrieval. Verify version-sensitive details at the source.