Regex Expression for not accepting a single underscore but accepts single alphabet

user_17022412 0
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The expression should not accept any special characters except an underscore and no numbers in it and it should accept single or more alphabets.
Examples

abc_dc (Should be accepted)

_bsc (Should be accepted)

A (Should be accepted)

_ (Should not be accepted)

a__b (should not be accepted)

I have tried:

(^(?!.*?[_]{2})[^_][a-zA-Z_]+$)

but this does not accept single alphabet.

regex
3 Answers

You could try following pattern:

^(?=.*[a-zA-Z])(?=[a-zA-Z_]+$)(?!.*_{2,}).+

Pattern explanation:

Generally, multiple validations are being achieved by placing lookaheads at beginning of a pattern, I used the same strategy.

Explanation:

^ - anchor - match beginning of a string

(?=.*[a-zA-Z]) - positive lookahead - assert that what follows current position is zero or more any characters (.*) followed by alphabet character. Simply - assert that what follows contains at least one letter.

(?=[a-zA-Z_]+$) - assert what follows contains only letters or underscore until end of string (thanks to $).

(?!.*_{2,}) - negative lookahead - assert what follows DOES NOT contain two (or more) following underscores (thanks to _{2,}

.+ - match on or more of any characters.

Regex demo

user_71325500

I got it.

(^(?!.*?[_]{2})[_]*[a-zA-Z]+[a-zA-Z_]*$)

This should work.

user_170224120

I think you could use:

^_?(?:[a-zA-Z]+_?)+$

See an online demo

  • ^ - Start line anchor.
  • _? - An optional hyphen.
  • (?: - Open non-capture group:
    • [a-zA-Z]+_? -Match 1+ alphachars and an optional hyphen.
    • )* - Close non-capture group and match 1+ times.
  • $ - End line anchor.

Note: This would also allow for trailing hyphens, e.g: '_a_', 'a_' and '_a_b_'. If you don't want to allow a trailing hyphen, try ^(?:_?[a-zA-Z]+)+$ instead.

user_97581940
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