regex - Grep regular expression for 4 digits among character string of variable length -

I need some methods to find words and any combination of digits but exactly 4 marks only , and at least one character

Example:

  a1a1a1a1 // match 1234 // no match (no letter) a1a1a1a1a1 // no match AB2b2 / no match CD12 / match Match 9999 / Match 1 AB 26A 9 // Match 1B1C1 // // Match Match 12345 / No Match 24 / No Match A2B2C2D2 / Match AB11CD22D3D3 / [
  

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To match a digit in GP [0-9] ] You can use. To match any number, you can use [^ 0- 9] because it can not be any number, or any character, so you add "*" (any number preceding it). So what do you want logically

(nothing a digit or nothing) * (no single digit) (nothing a digit or nothing) *. ...

Unless you have 4 "no single digits" group i.e. i.e. [^ 0- 9] * [0-9] ...

I am meeting with long patterns, especially with the long string of special characters, it is best to raise slowly, you are sure you understand what is going on. For example,

  # This will expose your matches, and it will otherwise make grep = 'grep --color = auto' echo 'a1b2' easier to understand. Grep '[0-9]'  

will show you how this match is going on. Once you understand each part you can expand the pattern.