Page 598 - Beginning PHP 5.3

Part III: Using PHP in Practice
Replacing Text using a Callback Function

preg_replace() is a powerful, flexible function, offering a multitude of ways to search and replace
text. However, if you need even more flexibility you can use preg_replace_callback() , which lets
you create a callback function to handle the replacement side of the operation.

preg_replace_callback() works in much the same way as preg_replace() , and accepts all the
same arguments, except that instead of passing a replacement string (or array of strings) as the second
argument, you pass the name of your callback function as a string.

Your callback function needs to accept an array of matches. The first element of the array (at index 0 )
contains the whole matched text, and additional elements contain any matched subpatterns. The string
that your function returns is then used as the replacement text.
Here ’ s an example. Say you have a large amount of sales copy that mentions prices of various products in
your online store, and you want to increase all your product prices by a dollar. You can ’ t do arithmetic
in regular expressions, but you can use preg_replace_callback() and a callback function to add
numbers together:
$text = “Our high-quality mouse mat is just $3.99,
while our keyboard covers sell for $4.99 and our
screen protectors for only $5.99.”;

function addADollar( $matches ) {
return “$” . ( $matches[1] + 1 );
}

echo preg_replace_callback( “/\\$(\d+\.\d{2})/”, “addADollar”, $text );

The addADollar() callback function takes the second element in the matches array, which contains the
matched text from the subpattern in the regular expression (that is, the price without the dollar symbol),
and adds one to it. It returns this new value, preceded by a dollar symbol. This string is then used by
preg_replace_callback() to replace the matched text, producing the following result:

Our high-quality mouse mat is just $4.99, while our keyboard covers sell for
$5.99 and our screen protectors for only $6.99.

Altering Matching Behavior with

Pattern Modifiers
By placing a single letter, known as a pattern modifier , directly after the closing delimiter of a regular
expression, you can change the way that the expression behaves. Here ’ s a list of the more useful
modifiers:

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