Regex To Validate A String, But The String Can't Contain n Number Of A Specific Character

user_11347887 0
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Recently I ran into a validation situation I've been trying to solve with regex. The rules are as such:

  • Must start with a capital letter
  • Center of the string may be of any length
  • Center of the string may have any combination of upper and lower case letters and numbers
  • Center of the string may have up to one underscore
  • Must end with a number

I have attempted to match this string with the following regex:

^(?!_{2,})([A-Z][a-zA-Z0-9_]*[0-9])$

and

^(?<=_{0,1})([A-Z][a-zA-Z0-9_]*[0-9])$

Both of these attempts still match cases where there is more than one underscore present. I.E. App_l_e9 or App__le9.

How can you check to see if your regex match, I.E. the ([A-Z][a-zA-Z0-9_]*[0-9]) part contains zero or one underscore in any place within the middle of the string?

regex
2 Answers

The simplest approach would probably be this

^[A-Z][a-zA-Z0-9]*_?[a-zA-Z0-9]*[0-9]$

Explanation:

  • ^[A-Z] Must start with an uppercase letter
  • [a-zA-Z0-9]* A combination of uppercase and lowercase letters and numbers of any length (also 0-length)
  • _? Either zero or one underscore character
  • [a-zA-Z0-9]* Again A combination of uppercase and lowercase letters and numbers of any length (also 0-length)
  • [0-9]$ Must end with a number

This will accept A_9 or AA0_xY8 but for instance not aXY_34 or Aasf1__asdf5

If the underscore in the middle part must not be the first or last character of this middlepart, you can replace the * with a + like this.

^[A-Z][a-zA-Z0-9]+_?[a-zA-Z0-9]+[0-9]$

So this, won't accecept for instance A_9 anymore, but the word must at least be Ax_d9

user_37769270

You might also start the match with an uppercase A-Z and immediately check that the string ends with a number 0-9 using a positive lookahead to prevent catastrophic backtracking.

^[A-Z](?=.*[0-9]$)[a-zA-Z0-9]*_?[a-zA-Z0-9]*$
  • ^ Start of string
  • [A-Z] Match an uppercase char A-Z
  • (?=.*[0-9]$) Positive lookahead to assert a digit 0-9 at the end of the string
  • [a-zA-Z0-9]* Optionally match any of the listed
  • _? Match an optional _
  • [a-zA-Z0-9]* Optionally match any of the listed
  • $ End of string

Regex demo

Or with an optional group

^[A-Z](?=.*[0-9]$)[a-zA-Z0-9]*(?:_[a-zA-Z0-9]*)?$

Regex demo

user_54249880
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