How to use look-ahead assertions

How to match assertions

Pattern	Meaning
(?=assertion)	Creates a look-ahead assertion
(?!assertion)	Creates a negative look-ahead assertion

Two subpatterns for look-ahead assertions

      (?=[[:digit:]])  // Assertion fails and returns 0
      (?=.*[[:digit:]]) // Matches and returns 1

How to create a look-ahead assertion

A look-ahead assertion

        $pattern = '/^(?=.*[[:digit:]])[[:alnum:]]{6}$/'; 
        preg_match($pattern, 'Harris'); // Assertion fails and returns 0
        preg_match($pattern, 'Harri5'); // Matches and returns 1

A negative look-ahead assertion

      $pattern = '/^(?!3[2-9])[0-3][[:digit:]]$/'; 
      preg_match($pattern, '32'); // Assertion fails and returns 0
      preg_match($pattern, '31'); // Matches and returns 1

A pattern to enforce password complexity

The full pattern

      $pw_pattern = '/^(?=.*[[:digit:]])(?=.*[[:punct:]])[[:print:]]{6,}$/';

The parts of the pattern

       ^  // From the start of the string
       (?=.*[[:digit:]])  // Must contain at least one digit
       (?=.*[[:punct:]]) // Must contain at least one punctuation character
       [[:print:]]{6,}  // Must contain six or more printable characters
       $ // With nothing else until the end of the string

Using the pattern

      $password1 = 'sup3rsecret'; 
      $password2 = 'sup3rse(ret'; 
      preg_match($pw_pattern, $password1); // Assertion fails and returns 0
      preg_match($pw_pattern, $password2); // Matches and returns 1

    Description
      A look-ahead assertion is a special type of subpattern that must be matched for the overall pattern to be matched.
A look-ahead assertion must be matched starting immediately after the position of the assertion in the pattern.
A negative look-ahead assertion is similar to a look-ahead assertion except that its pattern must not be matched.

    

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