Regular Expression

Regular expressions are a very powerful tool for searching and processing text, and even in their simplest form can make many seemingly complex tasks very easy.

Fortunately, Perl successfully integrates the power of regular expressions. Its flexible pattern matching ability gives it a great advantage over other languages (such as C) in the CGI world. For example, one statement is normally enough for Perl to process one string, while C would probably require many statements to implement the same task.

An example is the conversion of all the + (plus) signs in a string to the "space" sign. (Recall that input data are encoded before they are sent to the CGI program, where a space sign converts into a plus sign. After we receive the input data, we need to convert the plus sign back to its original space sign. This is usually the first step after receiving the input data from the clients). In Perl only one statement:

  $str=~tr/+//;

 void ADDToSpace(char *str)
  {
   register int i;
   for(i=0;str[i];i++)
     if(str[i]=='+'
        str[i]=' ';
 }

Syntax for regular expression

    /pattern/

Regular Expression Example

If this is all new to you, the above pattern table was probably quite confusing. Here are some examples:

Operators and Functions

• =~ (Pattern matching operator)

  Pattern matching operator( =~ )allows us to exampine scalar variables and test for the existence of a particular pattern in a string. Example:

    print "Please input a string: \n";
    $string=<STDIN>                   #accept an input string from standard input
    chop($string);                     #build-in function to chop the last newline character
    if($string=~/cgi/){
     print "The input string include substring cgi! \n";
    }else{
     print " The input string does not include substring cgi! \n";
    }
  

• !~ (Pattern not matching operator)

  Pattern not matching operator( !~ ) is the negative of Pattern matching operator(=~).

• /pattern/

  Let's take a look at serveral examples to see how it works:
  • Example 1

       $string="chmod 711 cgi";
       $string=~/(\w+)\s+(\d+)/;
     

    The (\w+) matches any number of characters. The matching substring will be assigned to variable $1. \s matches any number of spaces. The (\d+) matches any number of digits. $2 will get the matching result. So now $1="chmod"; $2="711". Note that the () is an important operator listed in the above table.

  • Example 2

      $_="chmod 711 cgi";
      /(\w+)\s+(\d+)/;  
     

    We will get the same result as Example 1. Note that if do not specify a operation string the default variable $_ will be used.

  • Example 3

     
        $string="chmod 711 cgi";
        @list=split(/\s+/,$string); #split string using space 
      

    Now we get:

     @list=("chmod","711","cgi");
      

• tr (Translation function)

  Syntax:

  tr/SEARCHLIST/REPLACELIST/

  Which translates SEARCHLIST to REPLACELIST. Here are two examples:

  1.         $string="testing";
             $string=~tr/et/ET;   #now $string="TEsTing"
             $string=~tr/a-z/A-Z/ #Here $string="TESTING"
      

  2.         $string="CGI+Perl";
             $string~tr/+//;   #Here $string=" CGI Perl"
      

• s (Substition function)

  Syntax:

  s/PATTERN/REPLACE/eg

  Which substitutes the PATTERN with the REPLACE pattern, where 'e' ang 'g' are the \ parameters:
  • 'g' means substitutes all the patterns matched by PATTERN in a string while omit it only substitutes the first occurence.
  • 'e' takes the REPLACE part as an equation instead of the common string.
  Here are some examples:
  • Example 1:

    $string="i:love:perl";
    $string=~s/:/*/;       # now $string="i*love:perl"
    $string=~s/:/*/;       # now $string="i*love*perl"
    $string=~s/*/+/;       # now $string="i+love+perl"
    $string=~s/+//g;       # now $string="i love perl"
    $string=~s/perl/cgi;   # now $string="i love cgi"   
     

  • Example 1:

    $string="i love perl";
    $string=~s/(love)/<$1>/; # now $string="i<love>perl" 
     # Here the first match "love" is assigned to $1 
    

  • Example 3:

    $string="www22cgi44";
    $string=~s/(\d+)/$1*2/e; #now $string="www44cgi44";
    #the paramater 'e' shows that the  $1*2 is an equation instead of a common string
     

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