SyntaxError: invalid capture group name in regular expression

The JavaScript exception "invalid capture group name in regular expression" occurs when a named capturing group or named backreference contains an invalid identifier.

Message

SyntaxError: Invalid regular expression: /(?<1>)/: Invalid capture group name (V8-based)
SyntaxError: invalid capture group name in regular expression (Firefox)
SyntaxError: Invalid regular expression: invalid group specifier name (Safari)

Error type

SyntaxError

What went wrong?

Each named capturing group must have a name that is a valid identifier. You cannot use arbitrary strings as the group identifier.

Examples

Invalid cases

js
/(?<1>\d+) (?<2>\d+)/;

Or you might be building the regex dynamically:

js
const tokenTypes = {
  "number literal": /\d+/,
  "string literal": /".+?"/,
  identifier: /[a-zA-Z_]\w*/,
};

const tokenPattern = new RegExp(
  Object.entries(tokenTypes)
    .map(([name, pattern]) => `(?<${name}>${pattern.source})`)
    .join("|"),
);

Valid cases

js
/(?<group1>\d+) (?<group2>\d+)/;

If the regex is built dynamically, make sure the names are all valid identifiers. For example:

js
const tokenTypes = {
  numberLiteral: /\d+/,
  stringLiteral: /".+?"/,
  identifier: /[a-zA-Z_]\w*/,
};

const tokenPattern = new RegExp(
  Object.entries(tokenTypes)
    .map(([name, pattern]) => `(?<${name}>${pattern.source})`)
    .join("|"),
);

See also