CODEWITHSUNDEEP
javaregex
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membersound

Reputation: 86687

How to restrict occurrence of a character in regex?

I want to check if a string consists of letters and digits only, and allow a - separator:

^[\w\d-]*$

Valid: TEST-TEST123

Now I want to check that the separator occurs only once at a time. Thus the following examples should be invalid:

Invalid: TEST--TEST, TEST------TEST, TEST-TEST--TEST.

Question: how can I restrict the repeated occurrence of the a character?

Upvotes: 2

Views: 136

Answers (1)

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 626738

You may use

^(?:[a-zA-Z0-9]+(?:-[a-zA-Z0-9]+)*)?$

Or, in Java, you may use an alphanumeric \p{Alnum} character class to denote letters and digits:

^(?:\p{Alnum}+(?:-\p{Alnum}+)*)?$

See the regex demo

Details

  • ^ - start of the string
  • (?: - start of an optional non-capturing group (it will ensure the pattern matches an empty string, if you do not need it, remove this group!)
    • \p{Alnum}+ - 1 or more letters or digits
    • (?:-\p{Alnum}+)* - zero or more repetitions of
      • - - a hyphen
      • \p{Alnum}+ - 1 or more letters or digits
  • )? - end of the optional non-capturing group
  • $ - end of string.

In code, you do not need the ^ and $ anchors if you use the pattern in the matches method since it anchors the match by default:

Boolean valid = s.matches("(?:\\p{Alnum}+(?:-\\p{Alnum}+)*)?");

Upvotes: 3

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