Regular Expressions

Parentheses as Memory

• Placing a set of parentheses ("()") around a part of a match expression allows you to "remember" that part of the match and reference it later.
• This will become of critical importance when we do substitution, but it is also usefull for matching operations.
• To reference a part of the expression in parens later in the same match expression, use a backslash ("\") followed by the number of the paren group you wish to match (counting from the left, starting with one).
• For example, to match "beeb", "peep", "deed", "teet", "feef"... we could use the expression "/(.)ee\1/".
• Example:
  

  # Find people with the same initial for their first, middle, 
  # and last names
  $theVar = "Lex Lee Luthor";
  if ($theVar =~ /(.).* \1.* \1.*/) {
      (print "Match One\n");  # Would Match
  }
  

• Example:
  

  # Find 10 character whatchamacallits
  $theVar = "amanaplanacanalpanama";
  if ($theVar =~ /(.)(.)(.)(.)(.)\5\4\3\2\1/) {
      (print "Match One\n");  # Would Match
  }