Question

I have string which contains all digits.

I want to check whether:

1. all the digits are in the range of 0-4, and
2. the minimum length of the string should be 5

So, I used this regex:

(?=^\d{5,}$)(^[0-4]*$) //works

As expected this does the job

But the above regex when used with positive look behind doesn’t work

(?<=^\d{5,}$)(^[0-4]*$) //doesn't work

Why does positive look behind doesn’t work but look ahead work’s in this case

EDIT

Yes,I can use

^[0-4]{5,}$

But the question is why look-behind didnt work in the above case

This was in reference to THIS question where lookahead worked but not lookbehind

Answer 1

Shouldn’t you look from behind, to match a “look behind assertion” ?

i.e.

(^[0-4]*$)(?<=^\d{5,}$)

As it stands now, you want this:

• following 5 or more characters (that comprise the entire string)
• the beginning of a string
• containing [0-4]

Which basically reduces to

• false