SyntaxError: invalid regexp group

The JavaScript exception "invalid regexp group" occurs when the sequence (? does not start a valid group syntax. Recognized group syntaxes that start with (? include:

(?: for non-capturing groups
(?= for positive lookahead
(?! for negative lookahead
(?<= for positive lookbehind
(?<! for negative lookbehind
(?< for named capturing groups
(?-i:, (?i:, (?m:, (?ims-:, etc. for modifiers

(? followed by any other character would cause this error.

Message

SyntaxError: Invalid regular expression: /(?1)/: Invalid group (V8-based)
SyntaxError: invalid regexp group (Firefox)
SyntaxError: Invalid regular expression: unrecognized character after (? (Safari)

Error type

SyntaxError

What went wrong?

? is not an atom, so it does not make sense to appear at the start of a capturing group (? is a quantifier and should be placed after an atom). Maybe you want to match the ? character literally, in which case you should escape it with a backslash (\?). Maybe you remembered the regex syntax wrong, and you intend to use one of the recognized group syntaxes listed above. Maybe you are using a feature that is not supported by the current JavaScript engine.

Examples

Invalid cases

/Hello(?|!)/;
// This is Perl syntax
/(?[\p{Thai}&\p{Digit}])/;

Valid cases

/Hello(\?|!)/;
// This is JavaScript syntax for character set operations
/[\p{Script=Thai}&&\p{Nd}]/v;