Regular Expression almost perfect for a Numeric Value

Question

I have this REGEX almost perfect ... It seems to handle everything except a number that leads with a negative sign and then a decimal. So if I enter:

-.2

I get an error -

Here is my Regex -- everything else I've tested works perfectly...

^(\+|-)?[0-9]{1,11}?(?:\.[0-9]{1,4})?$

This allows for:

• a number up to 11 digits (99 Billion)
• positive or negative number
• up to 4 decimal places (optional)
• leading 0 before decimal point is optional - for positive numbers only

These all work:

-0.2345
-10
12
.125
0.1245
5.555
25000000000 (aka 25 Billion)
25000000000.25 

These do not work:

-.2
-.421

Answer 1

Regex can be expensive... Why not use Decimal.Parse or Float.parse?

Your current implementation would never work with alternate number styles, like European where . (dot) and , (comma) are interchanged ...whereas Decimal.Parse will:

string stringValue = "45.889,33";
CultureInfo currentCulture = Thread.CurrentCulture; //set this way up in the execution chain
decimal thenumber = Decimal.Parse(stringValue, currentCulture);
//thenumber = 45889.33 in us-en display format.

Numerical parsing is not a good application for regex, IMO.

Answer 2

Try this:

^(\+|-)?[0-9]{0,11}?(?:\.[0-9]{1,4})?$

Update:

The above regex accepts strings +, - and (the empty string). You can use a lookahead to restrict those. The lookahead ensures there must be a character after the + or - sign.

The correct solution is:

^(\+|-)?(?=.{1})[0-9]{0,11}(?:\.[0-9]{1,4})?$

A working example:

Strings accepted:

-0.2345
-10
12
.125
0.1245
5.555
-.2
-.421

Strings not accepted:

100000000000.0001
+
-

123456789012
1111111111.12345
+1.11.1
-2.