Regex to match string of unique characters

Question

Here is my regular expression:

((GO)( [A-Z])+)

I want every letter to appear at most once, unfortunately it isn't working properly, because this input:

GO A B C C

return true, but should return false.

Answer 1

You can use this regex:

^(GO(?: ([A-Z])(?!.*\2))+)$

RegEx Demo

Answer 2

Your regex is:

GO(?:([A-Z])(?!.*\1))+$

• Matches the literal GO, followed by:
• Any character A-Z, from zero to infinite times
• asserting, for each character encountered, that the same character does not match any (.) subsequent character before the next line break ($).

The key to that last step, which is all you were missing, is the zero-length negative lookahead: (?!.*\1)

Answer 3

You could use the following regex:

^GO (?:([A-Z])(?!.*\1)\s*)*$

It will match anything that:

• starts with GO<space>
• contains only letters ([A-Z]) that:
  • may be separated by any sequence of blank characters, but:
  • may not have ever been seen before.

See it working on regex101!

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Sample matching cases:

GO A B C
GO ABC
GO A B C G O

Sample non-matching cases:

A B C
GO A A A