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Custom Tags Parsing Using Regular Expressions

In the last post, we had created a simple custom tag parsing script using PHP string functions. In this post, we are going to continue our discussion on custom tag parsing but rather using Regular Expressions. Here we will see how regular expressions can used to parse strings, we will also see where to and where not to use Regular Expressions. Before continuing, I expect that you have a working knowledge of Regular Expressions if not please first check out this websites.

Let us first create the previous custom tag parsing script using expressions:

<form name="form1" method="get" action="">
  <p>
    <!-- textarea should display previously wriiten text -->
    <textarea name="content" cols="35" rows="12" id="content"><? if (isset($_GET['content'])) echo $_GET['content']; ?></textarea>
  </p>
  <p>
    <input name="parse" type="submit" id="parse" value="Parse">
  </p>
</form>
<?

if(isset($_GET['parse']))
{
    
$content $_GET['content'];
    
//convert newlines in the text to HTML "<br />"
    //required to keep formatting (newlines)
    
$content nl2br($content);
    
    
//PHP function 'eregi_replace' replaces all occurences of the expression with the one mentioned
    //'\\1' is the string matched (one in parentheses '()' in the regular expression
    //it's a 'eregi_replace' thing not PHP's

    
$content eregi_replace('\.b\.(.+)\./b\.''<strong>\\1</strong>'$content);
    
$content eregi_replace('\.i\.(.+)\./i\.''<i>\\1</i>'$content);
    
    
//now the variable $content contains HTML formatted text
    //display it
    
echo '<hr />';
    echo 
$content;
}
?>

But should we use regular expressions here, answer is NO, because, first regular expressions run slower and they add a fair bit of complexity where the same thing could have been done easily using just string functions.

The reason for me staring this post with something contradicting to the theme of the post is because people tend to avoid regular expressions thinking that the same thing can be done otherwise (I just gave them one more chance!). Well it may be case sometimes but in many other cases where complex string manipulation is required with efficiency there is but one choice, regular expressions. The next example will illustrate this.

For this example we will parse ‘*’ (asterisk) and ‘_’ (underscore) for bolding and italicizing text (as in Google Talk / IM applications). The following text:

Hello *World*. Hello _World_.

Will be parsed and displayed as:

Hello World. Hello World.

It is quite obvious that both tags’ start and end tags are the same. Now let us see how this can be implemented (using regular expressions).

<form name="form1" method="get" action="">
  <p>
    <!-- textarea should display previously wriiten text -->
    <textarea name="content" cols="35" rows="12" id="content"><? if (isset($_GET['content'])) echo $_GET['content']; ?></textarea>
  </p>
  <p>
    <input name="parse" type="submit" id="parse" value="Parse">
  </p>
</form>
<?

if(isset($_GET['parse']))
{
    
$content $_GET['content'];
    
//convert newlines in the text to HTML "<br />"
    //required to keep formatting (newlines)
    
$content nl2br($content);
    
    
//match anything between the tags but not the tag itself
    //otherwise '*hello* world *hello*'
    //will be print 'hello* world *hello' in bold
    //and not 'hello(in bold) world hello(again in bold)'

    
$content eregi_replace('\*(.[^*]+)\*''<strong>\\1</strong>'$content);
    
$content eregi_replace('\_(.[^_]+)\_''<i>\\1</i>'$content);
    
    
//now the variable $content contains HTML formatted text
    //display it
    
echo '<hr />';
    echo 
$content;
}
?>

If we try to implement this using string functions it will take quite a lot more lines of extra coding but I leave that to you.

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