Annoying String to Float in AS3

Hey again,

I am working on *AS3* here.

Problem: I have a value stored in a String. I'd like to check whether
that String is a float. I am aware of the method parseFloat(string)
described here:
http://www.adobe.com/support/flash/action_scripts/actionscript_dictionary/
actionscript_dictionary620.html
However, this method has annoying limitations:
parseFloat("2.5") will be 2.5
parseFloat(" 2.5") will be 2.5
parseFloat("2.5garbage") will be 2.5
parseFloat("2.5garbage3.2") will be 2.5
it will only be NaN if the value of the string starts with a non-
numerical character
parseFloat("garbage2.5") will be NaN

So doing something like that, will not work for me:
public function isFloat : Boolean(str : String) {
if (isNaN(parseFloat(str))) {
return false;
}
return true;
}

I want to check if the string is a *real* float that is:
isFloat("2.5") will be true
isFloat("x2.5") will be false
isFloat("2.5x3.2") will be false
isFloat("2.5x") will be false
Basically, no spaces or non-numerical characters should be allowed
(expect the one dot '.' if required).

I'm thinking of implementing my own isFloat method, checking character by
character to see if the value is really a float or not. Something like
that (quick draft, haven't tested it or compiled it):

public function isFloat : Boolean (str : String) {
var dotUsed : Boolean = false;
for (var i : int = 0; i < str.length; ++i) {
if (str.charAt(i) == '.') {
if (!dotUsed) {
dotUsed = true;
} else {
return false;
}
} else if (isNaN(parseInt(str.charAt(i)))) {
return false;
}
}
}

However, I'm not happy with the runtime efficiency here. I'm sure there's
some more efficient way to detect that a string is a float; or another
simple method I haven't look at.

Any thoughts?

Thanks