regex - Validate a JavaScript function name

Question

What would be a regular expression which I can use to match a valid JavaScript function name...

E.g. myfunction would be valid but my<fun>ction would be invalid.

[a-zA-Z0-9_])?

Answer 1

This is more complicated than you might think. According to the ECMAScript standard, an identifier is:

an IdentifierName that is not a ReservedWord

so first you would have to check that the identifier is not one of:

instanceof typeof break do new var case else return void catch finally
continue for switch while this with debugger function throw default if
try delete in

and potentially some others in the future.

An IdentifierName starts with:

a letter
the $ sign
the _ underscore

and can further comprise any of those characters plus:

a number
a combining diacritical (accent) character
various joiner punctuation and zero-width spaces

These characters are defined in terms of Unicode character classes, so [A-Z] is incomplete. ? is a letter; ξ is a letter; 京 is a letter. You can use all of those in identifiers including those used for function names.

Unfortunately, JavaScript RegExp is not Unicode-aware. If you say w you only get the ASCII alphanumerics. There is no feasible way to check the validity of non-ASCII identifier characters short of carrying around the relevant parts of the Unicode Character Database with your script, which would be very large and clumsy.

You could try simply allowing all non-ASCII characters, for example:

^[_$a-zA-ZxA0-uFFFF][_$a-zA-Z0-9xA0-uFFFF]*$