phpmaster | Using PHP Regular Expressions

Core points

• Regular expressions (also known as regex) are patterns used to match text in strings. They are especially useful when you need to find text for different instances in a string.
• Regular expression notation uses special characters and symbols to define patterns. The "^" symbol specifies that the match must start at the beginning of the line, while " " is a quantifier that means that "at least one" of the previous character or collection must be matched.
• PHP uses functions such as preg_match(), preg_replace() and preg_match_all() to apply regular expressions. These functions can verify form fields, format text, and extract information arrays from strings, respectively.
• Metacharacters, quantifiers and separators play a crucial role in regular expression patterns. Understanding their capabilities can help you create more complex and precise search patterns.

^[A-Za-z0-9-_.+%]+@[A-Za-z0-9-.]+.[A-Za-z]{2,4}$

This code looks as hard to understand as ancient Egyptian hieroglyphs, but it is actually a regular expression pattern for matching email addresses such as oleomarg32@hotmail.com, Fiery.Rebel@veneuser.info, robustlamp selfmag@gmail.ca, etc. This article will introduce the basics of regular expressions and their application in PHP.

Beginner of regular expression notation

Let's break down the above example one by one: ^[A-Za-z0-9-_. %] @[A-Za-z0-9-.] .[A-Za-z]{2,4}$

• ^: Match the beginning of the string.
• [A-Za-z0-9-_. %] : Match one or more letters, numbers, or special characters (-_. %). Square brackets [] define character sets. The quantifier means "at least one".
• @: Literally match the "@" symbol.
• [A-Za-z0-9-.] : Match one or more letters, numbers, or dots (.).
• .: Literal match "." symbol (escaped).
• [A-Za-z]{2,4}: Match 2 to 4 letters. Braces {} Specifies the number of repetitions.
• $: Match the end of the string.

If you replace the first quantifier with *, for example:

^[A-Za-z0-9-_.+%]*@[A-Za-z0-9-.]+.[A-Za-z]{2,4}

can match strings like @SodaCanDrive.com because the * quantifier means "zero or more".

Regular expressions in PHP

After understanding regular expression notation, let's see how to use it in PHP. PHP provides several functions to handle regular expressions: preg_match(), preg_replace(), and preg_match_all().

preg_match()

preg_match() Used to check if there is a matching pattern in the string. Returns 1 if a match is found, otherwise returns 0.

<?php if (preg_match('/^[A-Za-z0-9-_.+%]+@[A-Za-z0-9-.]+.[A-Za-z]{2,4}$/', $_POST["emailAddy"])) {
    echo "Email address accepted";
} else {
    echo "Email address is all broke.";
}
?>

Note that regular expressions are wrapped with slash / as delimiter.

preg_replace()

preg_replace() Used to find and replace matching patterns.

^[A-Za-z0-9-_.+%]+@[A-Za-z0-9-.]+.[A-Za-z]{2,4}$

U modifier makes regular expressions match non-greedy. The brackets () are used to capture the matching text, and 1 is a backreference that refers to the first captured group.

preg_match_all()

preg_match_all() Used to find all matching patterns in a string and store the results into an array.

^[A-Za-z0-9-_.+%]*@[A-Za-z0-9-.]+.[A-Za-z]{2,4}

i modifier means case insensitive.

This article only introduces the basics of regular expressions, more advanced usages, such as prospects, backsights and more complex backreferences, please refer to the official PHP documentation.

Pictures from Boris Mrdja / Shutterstock

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