In Part 1 we had a quick look at what Perl and regular expressions
are, and introduced the idea of using them to process HTML files. In
this part we'll develop a Perl script to process an HTML file.
Suppose we have the following HTML file, called file1.htm:
Note:
Due to display considerations, 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" href="style.css"][/head]
[body bgcolor="white"]
[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]
Now
imagine that we want to change both occurrences of [h1]heading[/h1] to
[h1 class="big"]heading[/h1]. Not a major 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 what the script does.
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 one, 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.
Line 5
This line takes the contents of the $line variable and, via the OUT file handle, writes the line to new_file1.htm.
Line 6
This line closes the ‘while’ loop. The loop is repeated until all the lines in file1.htm have been examined.
Lines 7 and 8
These two lines close the two file handles that have been used in the
script. If you missed off these two lines the script would still work,
but it’s good programming practice to close file handles, thus freeing
up the file handle name so that it can be used, for example, by another
file.
Running the Script
As
the purpose of this article is to explain how to use regular
expressions to process HTML files, and not necessarily how to use Perl,
I don't want to dwell for too long on how to run Perl scripts. Suffice
to say that you can run them in various ways, for example, from within
a text editor such as TextPad, by double-clicking the perl script
(script1.pl), or by running the script from an MS-DOS window.
(The
location of the Perl interpreter will need to be in your PATH statement
so that you can run Perl scripts from any location on your computer and
not just from within the directory where the interpreter (perl.exe)
itself is installed.)
So, to run our script we could open an
MS-DOS window and navigate to the location where the script and the
HTML file are located. To keep life simple I've assumed that these two
files are in the same folder (or directory). The command to run the
script is:
C:>perl script1.pl
If the script does work
(and in theory it should), a new file (new_file1.htm) is created in the
same folder as file1.htm. If you open the file you’ll see the the two
lines that contained [h1] tags have been modified so that they now read
[h1 class="big"].
In Part 3 we'll look at how to handle multiple files.