function formattedMobileNum($numStr:String):String {
	var withNoSpaces:String = $numStr.split(" ").join("");
	var part1:String = withNoSpaces.substr(0, 5);
	var part2:String = withNoSpaces.substr(5, withNoSpaces.length-1);
	var formatted = part1 + " " + part2;
	formatted = formatted.replace(/^\s+|\s+$/g, "");
	return formatted;
}

var mobileNum:String = "      0 785 41 23 456  ";
trace(formattedMobileNum(mobileNum));
// OUTPUT
// 07854 123456

function formattedMobileNum($numStr:String):String {
	var withNoSpaces:String = $numStr.split(" ").join("");
	var part1:String = withNoSpaces.substr(0, 5);
	var part2:String = withNoSpaces.substr(5, withNoSpaces.length-1);
	var formatted = part1 + " " + part2;
	return formatted;
}

var mobileNum:String = "      0 785 41 23 456  ";
trace(formattedMobileNum(mobileNum));
// OUTPUT
// 07854 123456

This returns a string with a single space after the fifth character.
Useful for displaying a UK mobile number which is easy to read.

AS3 Format a UK Mobile Number (XXXXX XXXXXX)

number, format, mobile

ActionScript 3