Regex with a maximum value

Question

I have the following RegEx:

(^\d{1,3}$)|(\d{1,3})\.?(\d{0,0}[0,5])

This accepts any .5 increment of a number.

I want to add a range to this number of 13.5. Ideally, .5 should be valid to.

So, anything from .5 to 13.5, in .5 increments.

Examples allowed:

.5
4
12.5
13.5

Not allowed:

.56
3.45
14
14.5

Answer 1

Your current regex has some big issues e.g. [0,5] doesn't mean either 0 or 5 but a 0, , or 5. Try the following regex instead:

^(?=.)(?:\d|1[0-3])?(?:\.5)?$

See live demo here

Based on your given allowed examples it matches 12 but doesn't match 12.0. If you want to match 12.0 as well you have to replace \.5 with \.[05]:

^(?=.)(?:\d|1[0-3])?(?:\.[05])?$

Note: postive lookahead (?=.) ensures that an empty match doesn't occur.

Answer 2

^([0-9]|1[0-3])?(\.[05])?$

This works for your given examples