Perl | Regular Expressions

Regular Expression (Regex or Regexp or RE) in Perl is a special text string for describing a search pattern within a given text. Regex in Perl is linked to the host language and is not the same as in PHP, Python, etc. Sometimes it is termed as “Perl 5 Compatible Regular Expressions“. To use the Regex, Binding operators like ‘=~‘(Regex Operator) and ‘!~‘ (Negated Regex Operator) are used. Before moving to Binding operators let’s have a look at building patterns.

Building Patterns: In Perl, patterns can be constructed using the m// operator. In this operator, the required pattern is simply placed between the two slashes and the binding operators are used to search for the pattern in the specified string.

Using m// and Binding Operators: Mostly the binding operators are used with the m// operator so that required pattern could be matched out. Regex operator is used to match a string with a regular expression. The left-hand side of the statement will contain a string which will be matched with the right-hand side containing the specified pattern. Negated regex operator is used to check if the string is not equal to the regular expression specified on the right-hand side.

• Program 1: To illustrate the use of ‘m//’ and ‘=~’ as follows:

  # Perl program to demonstrate
  # the m// and =~ operators
  
  # Actual String
  $a = "GEEKSFORGEEKS";
  
  # Prints match found if
  # its found in $a
  if ($a =~ m[GEEKS])
  {
      print "Match Found\n";
  }
  
  # Prints match not found
  # if its not found in $a
  else
  {
      print "Match Not Found\n";
  }
  Output:

  Match Found
  

• Program 2: To illustrate the use of ‘m//’ and ‘!~’ as follows:

  # Perl program to demonstrate
  # the m// and !~ operators
  
  # Actual String
  $a = "GEEKSFORGEEKS";
  
  # Prints match found if
  # its not found in $a
  if ($a !~ m[GEEKS])
  {
      print "Match Found\n";
  }
  
  # Prints match not found
  # if it is found in $a
  else
  {
      print "Match Not Found\n";
  }
  Output:

  Match Not Found
  

Uses of Regular Expression:

• It can be used to count the number of occurrence of a specified pattern in a string.
• It can be used to search for a string which matches the specified pattern.
• It can also replace the searched pattern with some other specified string.