SyntaxError: invalid range in character class

The JavaScript exception "invalid range in character class" occurs when a character class in a regular expression uses a range, but the start of the range is greater than the end.

Message

SyntaxError: Invalid regular expression: /[2-1]/: Range out of order in character class (V8-based)
SyntaxError: invalid range in character class (Firefox)
SyntaxError: Invalid regular expression: range out of order in character class (Safari)

Error type

SyntaxError

What went wrong?

In character classes, you can join two characters with a hyphen - to represent an inclusive interval of characters based on their Unicode code points. For example, [a-z] matches any lowercase letter. However, if the end of the range is less than the start, the range cannot match anything and is likely a mistake.

Examples

Invalid cases

js
/[2-1]/; // The range is out of order
/[_-=]/; // _ has value 95, = has value 61

Valid cases

js
/[1-2]/; // Swap the range
/[_\-=]/; // Escape the hyphen so it matches the literal character

See also