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Invalid regular expression

Fix "Invalid regular expression"

JavaScript rejected the pattern before running it: an unbalanced bracket or paren, a dangling quantifier, a bad escape, or an unknown flag.

Input that triggers it

(\d{3})-(\d{4}
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What the error means

Before a regular expression can match anything, the engine compiles it. Invalid regular expression: /.../: <reason> is JavaScript's compile-time failure: the pattern violates the grammar, so no matching is attempted at all. The part after the colon is the useful bit — Unterminated group, Unmatched ')', Nothing to repeat, Invalid escape, Unterminated character class, Invalid flags — and it names the specific rule that was broken.

In the example, the second group opens with ( and never closes, so the reason is Unterminated group.

Why it happens

  • Unbalanced brackets. A ( without ), a [ without ], or a { that starts a quantifier and never closes. Easy to do when editing a long pattern by hand.
  • Nothing to repeat. A quantifier with nothing before it: a pattern that starts with * or +, or a group like (?+). Also a**, two quantifiers in a row.
  • A literal special character. ., +, ?, (, [, {, |, ^, $, \ all have meaning. Matching a literal ( requires \(; matching 1.5 literally requires 1\.5.
  • A bad escape. \p{Letter} without the u flag, or \k<name> referring to a group that does not exist.
  • Invalid or duplicate flags, such as gg or a flag the engine does not support.
  • Pattern copied from another language. Python's (?P<name>...), PCRE's possessive a++, and .NET's (?<name>...) with different lookbehind rules do not all translate to JavaScript.

How to fix it

  1. Paste the pattern into the tester above. The error card repeats the engine's reason; read the words after the colon.
  2. For unbalanced brackets, count them, or format the pattern with one group per line in your head: every ( needs a ).
  3. For "nothing to repeat", find the quantifier at the start of the pattern or group and either remove it or give it something to repeat.
  4. For a literal special character, prefix it with \. String Escape in regex mode escapes a whole literal string at once, which is safer than doing it by eye.
  5. Compile again. Once the pattern is valid, the match table shows what it captures.

The corrected example:

(\d{3})-(\d{4})

If it still fails

  • A pattern built from user input needs escaping before it becomes a regex; a stray ( from the user produces exactly this error at runtime.
  • If the pattern is valid but slow or never returns, that is a different problem — see Catastrophic backtracking.
  • Regex Generator builds common patterns (emails, dates, IPs) with the escaping already correct, which is a faster starting point than a blank editor.

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