Regular Expressions in PHP
In this lesson: Learn regular expressions in PHP — preg_match, preg_replace, preg_split — with email, mobile and pin-code patterns.
What is regex
A regular expression (regex) is a pattern for matching text. Instead of checking characters one by one, you describe the shape you want — "digits only", "starts with A", "an email format" — and PHP finds or changes matching text. It is powerful for validation and search-and-replace.
/pattern/. Add i after for case-insensitive.preg_match to test
<?php
$pin = "1234";
if (preg_match('/^\d{4}$/', $pin)) {
echo "Valid 4-digit PIN";
} else {
echo "Invalid";
}
?>The pattern /^\d{4}$/ means: start, exactly 4 digits, end. preg_match returns 1 if the whole string fits — perfect for format checks.
Common pattern symbols
| Symbol | Means |
|---|---|
| \d | Any digit (0-9) |
| \w | Letter, digit or underscore |
| . | Any single character |
| + | One or more of the previous |
| {4} | Exactly 4 of the previous |
| ^ and $ | Start and end of the string |
preg_replace to change
<?php
$text = "Call 98765 or 12345";
// replace every run of digits with #
echo preg_replace('/\d+/', '#', $text);
?>preg_replace swaps every part that matches the pattern with new text. Here every group of digits becomes a # — handy for masking or cleaning text.
preg_match_all
To get every match, not just the first, use preg_match_all.
<?php
$text = "Order 12 and 34 and 56";
preg_match_all('/\d+/', $text, $matches);
print_r($matches[0]);
?>All the numbers are collected into $matches[0]. Use this when a string can contain many things you want to pull out.
Common mistakes
- Forgetting the delimiters. A pattern must be wrapped, e.g.
/\d+/, not just\d+. - Missing ^ and $. Without them a partial match passes; add both to check the whole string.
- Writing a regex for email. Use
filter_varfor email — it is more reliable. - Not escaping special characters. To match a literal dot, write
\., since.means any character.
Summary
- Regex is a pattern for matching or changing text.
preg_matchtests if text fits a pattern (returns 1/0).preg_replaceswaps matching text for something new.preg_match_allcollects every match.- Wrap patterns in delimiters and use
^/$for whole-string checks.
Next, validate input with form validation, or clean strings with PHP string functions.
Frequently Asked Questions
What is a regular expression in PHP?
preg_match and preg_replace with these patterns.What does preg_match return?
preg_match returns 1 if the pattern is found, 0 if not, and false on error. It is used to test whether text matches a pattern, such as validating a format.Why are patterns wrapped in slashes like /.../?
/^\d+$/. Anything after the closing slash can be a flag, like i for case-insensitive matching.What is the difference between preg_match and preg_match_all?
preg_match stops at the first match. preg_match_all finds every match in the string and collects them all. Use the "all" version when you expect more than one result.Should I use regex to validate an email?
filter_var($email, FILTER_VALIDATE_EMAIL) is more reliable than a hand-written regex. Use regex for simpler custom patterns like a PIN code or a username format.