Regex to test for lowercase characters

Ai-Lyn Tang

Feb 7, 2019·2 min read

Regular expressions. So powerful, but so tricky to learn!

Recently I found myself scratching my head over how to write a regular expression to check that a string contained only lowercase characters.

I used so many different combinations of[a-z], ^\W, [0–9], [a-z]^\W and more. No dice. I couldn’t figure out the correct combination. Finally I thought: oh, so simple, I’ll just do the inverse! So I did the following:

const isLowercaseOnly = (string) => {
  const nonLowercaseLetters = /[^a-z]/g;
  return !nonLowercaseLetters.test(string);
};

This works, but now I have to deal with inverse. A bit confusing.

Luckily, I was working with a great mentor from Exercism who helped me out with the expression. It is ^[a-z]+$. To explain:

• ^ : start the string with something from [a-z]
• +: there can be one or more occurrences from the range [a-z]. Without this, the regex would only be true for strings with only one lowercase character present. e.g. ‘h’ would be true, but ‘hello’ would be false.
• $: the string must end with something from [a-z]
• As the expression only contains one range between the ^ and $, no other characters are permitted. So you don’t need to worry about digits, white spaces, uppercase etc. creeping in because the expression has to start and end with [a-z].

So my final function is:

const isLowercaseOnly = (string) => {
  const lowercaseOnly = /^[a-z]+$/g;
  return lowercaseOnly.test(string);
}

Ta-dah! No more negatives. I also found this website to be incredibly powerful to test regex combinations. Thank you to whomever created it!