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

[EDIT] See @bobince’s post below for a more correct and thorough answer. This answer has been retained for reference and edited to be less wrong.

A valid name in JavaScript must start with a Unicode letter (\p{L}), dollar sign, or underscore then can contain any of those characters as well as numbers, combining diacritical (accent) characters, and various joiner punctuation and zero-width spaces. Additionally, it cannot be a word reserved by the JavaScript language (e.g. abstract, as, boolean, break, byte, case, etc).

A full regular expression solution would be quite complicated in plain JavaScript but the XRegExp Unicode plugin could greatly simplify the task. This online function name tester might also be useful.

[ORIGINAL] Here is an incomplete regular expression, using only the US ASCII letters:

var fnNameRegex = /^[$A-Z_][0-9A-Z_$]*$/i;

You also must check that it doesn’t match any reserved words (e.g. abstract, boolean, break, byte, …, while, with, etc). Here’s a start for that list, and an example function:

var isValidFunctionName = function() {
  var validName = /^[$A-Z_][0-9A-Z_$]*$/i;
  var reserved = {
    'abstract':true,
    'boolean':true,
    // ...
    'with':true
  };
  return function(s) {
    // Ensure a valid name and not reserved.
    return validName.test(s) && !reserved[s];
  };
}();