Regex to match a string containing as many digits as letters

Question

I am trying to create a regex to match a string with the following criterion:

1. the string must contain an even number of chars
2. the string must contain as many digits as letters

Should match:

• A3D4
• A34DF5
• 22FF

I tried, but didn't get a solution. Can you help me out?

Answer 1

If the programming language you're using supports regex subroutines, then the following one should suit your needs:

^([A-Z](?1)*[0-9]|[0-9](?1)*[A-Z])+$

Visualization by Debuggex

Demo on regex101

Answer 2

using regexp to keep only letters or only numbers and comparing them : (example in javascript)

var str = 'A34DF5';
var result = str.replace(/[^a-z]/gi,'').length == str.replace(/[^0-9]/gi,'').length ;