RegEx for matching 3 alphabets and 1-2 digits

Question

I am trying to write a regular expression to find a match in a text having at least 100 characters. The match should be like this - Any sub string within a string that contains at least 3 alphabet to begin with, at least 1 digit following it and a maximum of 2 digits following the 3 letters.

Examples -

1. abcjkhklfdpdn24hjkk - In this case I want to extract pdn24

2. hjdksfkpdf1lkjk - In this case I want to extract pdf1

3. hjgjdkspdg34kjfs dhj khk678jkfhlds1 - In this case I want both pdg34 and lds1

How do I write a regex for this ? The length of the starting letters for a match is always 3 and the digits length can be either 1 or 2 (not more not less)

This is what works if there are 2 digits after the 3 letter string.

[A-Za-z]{3}[0-9]{2}

But the length of the digits can vary between 1 and 2. How do I include the varying length in the regex?

Answer 1

The expression we wish to design is quite interesting. We can first add your original expression with a slight modification in a capturing group, then we should think of left and right boundaries around it. For instance, on the right we might want to use \D:

([A-Za-z]{3}[0-9]{1,2})\D

DEMO 1

We can surely define an exact restricted expression. However, this might just work.

---

Based on Cary Swoveland's advice, we can also use this expression, which is much better:

\p{L}{3}\d{1,2}(?!\d)

Test

re = /([A-Za-z]{3}[0-9]{1,2})\D/m
str = 'abcjkhklfdpdn24hjkk
hjdksfkpdf1lkjk
hjgjdkspdg34kjfs dhj khk678jkfhlds1 '

# Print the match result
str.scan(re) do |match|
    puts match.to_s
end

This script shows how the capturing group works:

const regex = /([A-Za-z]{3}[0-9]{1,2})\D/gm;
const str = `abcjkhklfdpdn24hjkk
hjdksfkpdf1lkjk
hjgjdkspdg34kjfs dhj khk678jkfhlds1 `;
let m;

while ((m = regex.exec(str)) !== null) {
    // This is necessary to avoid infinite loops with zero-width matches
    if (m.index === regex.lastIndex) {
        regex.lastIndex++;
    }
    
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}