PHP program to remove non-alphanumeric characters from string

To remove non-alphanumeric characters from a string in PHP, you can use the preg_replace() function with regular expressions. This function searches for a pattern and replaces it with specified content.

Method 1: Using [^a-z0-9] Pattern

The most common approach uses a negated character class to match anything that's not alphanumeric −

<?php
$my_str = "Thisis!@sample*on&ly)#$";
$my_str = preg_replace('/[^a-z0-9]/i', '', $my_str);
echo "The non-alphanumeric characters removed gives the string as ";
echo $my_str;
?>

The non-alphanumeric characters removed gives the string as Thisissampleonly

The pattern /[^a-z0-9]/i means:

• [^a-z0-9] − Match any character that is NOT a lowercase letter or digit
• i flag − Makes the pattern case-insensitive

Method 2: Using \W Pattern

The \W pattern matches non-word characters (anything except letters, digits, and underscores) −

<?php
$my_str = "This!#is^&*a)(sample*+_only";
$my_str = preg_replace('/\W/', '', $my_str);
echo "The non-alphanumeric characters removed gives the string as ";
echo $my_str;
?>

The non-alphanumeric characters removed gives the string as Thisisasample_only

Note: The \W pattern preserves underscores since they're considered word characters, while [^a-z0-9] removes them.

Method 3: Strict Alphanumeric Only

To remove underscores as well and keep only letters and numbers −

<?php
$my_str = "Hello_World123!@#";
$my_str = preg_replace('/[^A-Za-z0-9]/', '', $my_str);
echo "Strict alphanumeric result: " . $my_str;
?>

Strict alphanumeric result: HelloWorld123

Comparison

Conclusion

Use /[^a-z0-9]/i for strict alphanumeric filtering or /\W/ if you want to preserve underscores. Both methods effectively clean strings by removing unwanted special characters.