Regex to match all capital and underscore

Question

I need a Regex to test a string whether match the follow rules:

1. Contains at least a word (could be only a character)
2. All characters should be capital.
3. Use one, and only one, underscore (_) between each word pairs (e.g. HELLO_WOLRD)

The test values (valid and invalid):

const validConstants = [
  'A',
  'HELLO',
  'HELLO_WORLD',
];
const invalidConstants = [
  '',               // No empty string
  'Hello',          // All be Capitals
  'Add1',           // No numbers
  'HelloWorld',     // No camel cases
  'HELLO_WORLD_',   // Underscores should only be used between words
  '_HELLO_WORLD',   // Underscores should only be used between words
  'HELLO__WORLD',   // Too much Underscores between words
];

I tried ^[A-Z]+(?:_[A-Z]+)+$, but it fails in A and HELLO.

Answer 1

You need a * quantifier at the end:

^[A-Z]+(?:_[A-Z]+)*$
                  ^ 

The (?:_[A-Z]+)* will match zero or more sequences of _ and 1 or more uppercase ASCII letters.

See the regex demo.

Details:

• ^ - start of string anchor
• [A-Z]+ - 1+ uppercase ASCII letters (the + here requires at least one letter in the string)
• (?:_[A-Z]+)* - a non-capturing group matching zero or more sequences of:
  • _ - an underscore
  • [A-Z]+ - 1+ uppercase ASCII letters (the + here means the string cannot end with _)
• $ - end of string anchor