SyntaxError: invalid property name in regular expression

The JavaScript exception "invalid property name in regular expression" or "invalid class property name in regular expression" occurs when the \p and \P Unicode character class escapes are not followed by a valid Unicode property name and/or value.

Message

SyntaxError: Invalid regular expression: /\p{x}/u: Invalid property name (V8-based)
SyntaxError: Invalid regular expression: /[\p{x}]/u: Invalid property name in character class (V8-based)
SyntaxError: invalid property name in regular expression (Firefox)
SyntaxError: invalid class property name in regular expression (Firefox)
SyntaxError: Invalid regular expression: invalid property expression (Safari)

Error type

SyntaxError

What went wrong?

In Unicode-aware mode, the \p and \P escape sequences are used to match characters or character sequences based on their Unicode properties. The \p escape sequence matches characters with the specified Unicode property, while the \P escape sequence matches characters without the specified Unicode property. The syntax is:

regex
\p{loneProperty}
\P{loneProperty}

\p{property=value}
\P{property=value}

If it's not followed by {, then a recognizable property name/value, then }, this syntax error is thrown. For more information on what lone properties and property names are allowed, see the Unicode character class escape reference.

Examples

Invalid cases

js
/\p{x}/u; // "x" is not a valid Unicode property name
/\p{Script=x}/u; // "x" is not a valid value for Script
/\property/u; // "\p" is not followed by the right syntax
/\p{RGI_Emoji_Flag_Sequence}/u;
// "RGI_Emoji_Flag_Sequence" is a property of strings, so the "v" flag must be used
/\P{RGI_Emoji_Flag_Sequence}/v; // \P cannot match properties of strings

Valid cases

js
/\p{Script=Latin}/u; // "Script=Latin" is a valid Unicode property
/\p{Letter}/u; // "Letter" is valid value for General_Category
/\p{RGI_Emoji_Flag_Sequence}/v; // Property of strings can only be used in "v" mode

See also