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

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.