Using Perl and Regular Expressions to Process Html Files - Part 2

In this article we will discuss how to change the contents of an HTML file by running a Perl script on it.

The file we are going to process is called file1.htm:

Note: To ensure that the code is displayed correctly, in the example code shown in this article, square brackets '[..]' are used in HTML tags instead of angle brackets ''.

[html]
[head][title]Sample HTML File[/title]
[link rel="stylesheet" type="text/css" onClick="javascript:pageTracker._trackPageview('/outgoing/article_exit_link');" href="style.css"]
[/head]
[body]
[h1]Introduction[/h1]
[p]Welcome to the world of Perl and regular expressions[/p]
[h2]Programming Languages[/h2]
[table border="1" width="400"]
[tr][th colspan="2"]Programming Languages[/th][/tr]
[tr][td]Language[/td][td]Typical use[/td][/tr]
[tr][td]JavaScript[/td][td]Client-side scripts[/td][/tr]
[tr][td]Perl[/td][td]Processing HTML files[/td][/tr]
[tr][td]PHP[/td][td]Server-side scripts[/td][/tr]
[/table]
[h1]Summary[/h1]
[p]JavaScript, Perl, and PHP are all interpreted programming languages.[/p]
[/body]
[/html]

Imagine that we need to change both occurrences of [h1]heading[/h1] to [h1 class="big"]heading[/h1]. Not a big change and something that could be easily done manually or by doing a simple search and replace. But we're just getting started here.

To do this, we could use the following Perl script (script1.pl):

1 open (IN, "file1.htm");
2 open (OUT, ">new_file1.htm");
3 while ($line = [IN]) {
4 $line =~ s/[h1]/[h1 class="big"]/;
5 (print OUT $line);
6 }
7 close (IN);
8 close (OUT);

Note: You don't need to enter the line numbers. I've included them simply so that I can reference individual lines in the script.

Let's look at each line of the script.

Line 1
In this line file1.htm is opened so that it can be processed by the script. In order to process the file, Perl uses something called a filehandle, which provides a kind of link between the script and the operating system, containing information about the file that is being processed. I've called this "opening" filehandle 'IN', but I could have used anything within reason. Filehandles are normally in capitals.

Line 2
This line creates a new file called 'new_file1.htm', which is written to by using another filehandle, OUT. The '>' just before the filename indicates that the file will be written to.

Line 3
This line sets up a loop in which each line in file1.htm will be examined individually.

Line 4
This is the regular expression. It searches for one occurrence of [h1] on each line of file1.htm and, if it finds it, changes it to [h1 class="big"].

Looking at Line 4 in more detail:

• $line - This is a variable that contains a line of text. It gets modified if the substitution is successful.





• =~ is called the comparison operator.





• s is the substitution operator.





• [h1] is what needs to be substituted (replaced).





• [h1 class="big"] is what [h1] has to be changed to.