regular expression to allow only numbers in range 1-20

Question

I want to make user to enter numbers in 0-20 range and they can both enter 01 and 1

this is what I have so far

 /^[1-9]|0[1-9]|1[0-9]|2[0]$/

but it doesn't work.

Answer 1

The problem is that | has lower precedence than ^ and $, so your pattern means ^[1-9] or 0[1-9] or 1[0-9] or 2[0]$: only single-digit values are restricted by ^, and only 20 is restricted by $.

You can either repeat ^ and $ in each branch of the alternation, or else wrap the alternation in (?:...) to create a non-capturing subexpression:

/^[1-9]$|^0[1-9]$|^1[0-9]$|^20$/
/^(?:[1-9]|0[1-9]|1[0-9]|20)$/

(I've also taken the liberty of changing [0] to 0.)

Answer 2

try this

/^([01]?\d|20)$/

0 or 1 (optional) followed by at least one digit OR 20