fjordtools

fjordtools / Errors / Regex: Unterminated group

SyntaxError: Invalid regular expression: Unterminated group

The pattern opens a group with a left parenthesis that never closes, so the engine reaches the end of the pattern still inside the group. A stray right parenthesis gives the matching error Unmatched ). Count the pairs, and escape a literal parenthesis with a backslash.

Fjordtools team · Seen in Chrome and Node (V8) · Published · 1 min read

How browsers word it

Chrome and Node (V8)Invalid regular expression: /(abc/: Unterminated group
FirefoxSyntaxError: unterminated parenthetical
SafariSyntaxError: Invalid regular expression: missing )

Common causes

  1. A group was never closed

    A left parenthesis opens a group and every group needs its closing one.

    Input

    (abc

    Regex Tester says

    Invalid regular expression: /(abc/: Unterminated group

  2. An extra right parenthesis

    A closing parenthesis with no group to close is reported as unmatched.

    Input

    abc)

    Regex Tester says

    Invalid regular expression: /abc)/: Unmatched ')'

  3. A square bracket was never closed

    A character class has the same problem with its brackets.

    Input

    [a-z

    Regex Tester says

    Invalid regular expression: /[a-z/: Unterminated character class

How to fix it

  1. Count the opening and closing parentheses, and indent nested groups while you read.
  2. Escape a literal parenthesis or bracket with a backslash: \( and \).
  3. When you build the pattern from user input, escape the input first.
  4. Paste the pattern into the regex tester to see the exact error.

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