SyntaxError: invalid named capture reference in regular expression

The JavaScript exception "invalid named capture reference in regular expression" occurs when a regular expression pattern contains a named backreference that does not refer to a named capture group before it. The similar error message "invalid named reference in regular expression" is thrown when the sequence \k is encountered but is not followed by <.

Message

SyntaxError: Invalid regular expression: /\k<x>/u: Invalid named capture referenced (V8-based)
SyntaxError: invalid named capture reference in regular expression (Firefox)
SyntaxError: Invalid regular expression: invalid \k<> named backreference (Safari)

SyntaxError: Invalid regular expression: /\k/u: Invalid named reference (V8-based)
SyntaxError: invalid named reference in regular expression (Firefox)
SyntaxError: Invalid regular expression: invalid escaped character for Unicode pattern (Safari)

Error type

SyntaxError

What went wrong?

You are using the escape sequence \k which is parsed as a named backreference, but the \k is either syntactically invalid or does not refer to a named capture group in the pattern.

\k only starts a named backreference when the pattern contains a named capturing group or when the pattern is Unicode-aware; otherwise, it is an identity escape for the character k.

Examples

Invalid cases

js
/\k<x>/u; // Does not refer to a named capture group
/(?<x>.+)\k<y>/u; // Does not refer to an existing named capture group
/(?<x>.+)\k{x}/u; // Invalid syntax

Valid cases

js
/(?<x>.+)\k<x>/u; // Refers to an existing named capture group

See also