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JavaScript regular expression used an invalid flag

SyntaxError: invalid regular expression flag "x"

JavaScript rejected a regular expression because one of the supplied flags is unsupported or malformed.

SyntaxError: invalid regular expression flagregex

Observed message

SyntaxError: invalid regular expression flag "x"

Why it happens

  • The regular expression uses a flag the runtime does not support.
  • A typo or duplicated flag was added to the pattern.
  • Dynamic regex construction produced an invalid flags string.

How to fix it

  1. Use only flags supported by the current JavaScript runtime.
  2. Check for typos or repeated flags in the second RegExp argument.
  3. Log dynamically generated regex flags before creating the expression.

Official reference

MDN: SyntaxError: invalid regular expression flag x

Related errors

SyntaxError: nothing to repeatJavaScript regular expression has nothing to repeatReferenceError: can't access lexical declaration before initializationJavaScript accessed a let or const binding before initializationReferenceError: assignment to undeclared variableJavaScript assigned to an undeclared variableTypeError: BigInt value can't be serialized in JSONJavaScript BigInt value cannot be serialized in JSON

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JavaScript invalid regular expression flag: causes and fixes | Error Atlas