A regular expression that matches any legal JavaScript number, including weird edge cases. If you have a failing test case, let me know!

numberRegex.js

var pattern = /(?:[+-]?)0*(?:(?:(?:[1-9]\d*)?\.\d+)|(?:(?:0|[1-9]\d*)\.)|(?:0|[1-9]\d*))(?:[eE][+-]?\d+)?/;

var shouldPass = [
	'0',
	'00',
	'000123',
	'0.01',
	'1',
	'-0',
	'+1',
	'1.234',
	'.234',
	'-.234',
	'-0.234',
	'4.'
];

var shouldFail = [
	'',
	'.',
	'foo',
	'0.1.2',
	'1,234'
];

shouldPass.forEach( function ( testcase ) {
	var match = pattern.exec( testcase );
	console.assert( match && testcase === match[0], '"' + testcase + '"' );
});

shouldFail.forEach( function ( testcase ) {
	var match = pattern.exec( testcase );
	console.assert( !match || testcase !== match[0], '"' + testcase + '"' );
});

NB: this doesn't include hex numbers, and it allows signs before numbers (which isn't technically part of ECMA-262 but would be a nuisance to omit).