Regex for floating point? - ruby

Question

I'm trying to write a RegEx to validate a floating point number. Here's what I've managed thus far:
/^[-+]?[1-9]\d{0,2}(\.\d{1,1})?/
The number is valid if:
Either positive or negative
Max of 2 digits (tens or hundreds)
Hundredths digit can't 0 (only 1-9)
Scale is max of 1
Decimal value can be 0 or 5 or none at all
So these numbers would be valid, for ex:
1.5
-1.5
17.5
15
-3
30.5
These numbers would be invalid, for ex:
1.57
3041.5
17.59
915
-1.56
05.0

Answer 1

How about Float(number) (catch exceptions to detect parse errors) and then verify the floating point number? This will be easier for several of the properties than writing a regex. If you need to force a subset of the syntax accepted by Ruby to be used (why?), check only that part with a regex.

Answer 2

As I said in the comments - your first non-zero digit will count, then you add two more - which will now allow three-digit numbers like 915. To solve this the regexp way (with your testcases):
^[+-]?(?:(?!0)\d{1,2}|0)(?:\.[05])?$
I use negative lookahead (?!0) to make sure the first digit is not a zero, then just require the desired number of digits. It also allows 0.5 and similar through the |0 disjunction. If you prefer .5, it'll be this:
^[+-]?(?!0)\d{,2}(?:\.[05])?$
If you want to disallow 3.0 (allowed by your rules) and only allow 3 (as you imply in the examples), replace the last part:
^[+-]?(?:(?!0)\d{1,2}|0)(?:\.5)?$
However, this is much less readable than #Arkku's nice Float(number); use regexps if you really need them.

Answer 3

Here's a whole webpage on validating floats in regular expressions: http://www.regular-expressions.info/floatingpoint.html
That said,
{1,1} is equivalent to {1} which is equivalent to not putting it at all, so you can remove it
If you want to make sure that 0.5xyz doesn't pass, then add a $ at the end
With the above changes, it'd look like this: /^[-+]?[1-9]\d{0,2}(\.\d)?$/

Answer 4

According to your valid and invalid examples on: Regex for floating point?
This regex would work:
^[+-]?([1-9][0-9]?|0)(\.[05])?$

Answer 5

/^[-+]?\d{0,2}(\.[05])?(?!\d)$/
Matches when:
Value is either positive or negative
magnitude is less than 100. in the range of (-99.5 to 99.5)
Decimal value is .0 or .5 or absent (in other words, .0 or .5, or missing)
So these numbers would be valid, for ex:
1.5
-1.5
17.5
15
-3
30.5
-10.0
99.5
These numbers would be invalid, for ex:
1.57
3041.5
17.59
915
-1.56
05.0
99.6