PHP
Matching with preg_match()
Learn to match a pattern in a string with preg_match().
What are regular expressions (regex)?
Regular expressions (regex) are a small but powerful language for searching patterns inside text. In PHP, regex patterns are written between two delimiters; the most common delimiter is the slash: /apple/.
To match a pattern against text you use preg_match($pattern, $subject, $matches). It returns 1 if a match is found, 0 otherwise; the matched piece is written into the $matches array.
Inside a pattern, letters and digits match themselves; special characters carry meaning.
preg_match parameters and $matches
preg_match takes three main parameters: the pattern, the text to search, and the array to hold matches. The third parameter, $matches, is optional but used most of the time.
If a match is found, $matches[0] holds the whole matched piece. If the pattern uses parentheses for capture groups, $matches[1], $matches[2] hold the groups in order.
If no match is found, the array stays empty. Since the return value is numeric, the if (preg_match(...)) idiom is common.
Exercise Find the first digit
In the string $phone = "Tel: 0555 123 45 67";, use preg_match to find the first digit. Put the pattern into $pattern, the result into $matches, and print $matches[0] on a new line.
Show example solution
Set the pattern to "/\d/". Call preg_match($pattern, $phone, $matches). Print with echo $matches[0] . "\n";
Example solution:
<?php
// Find the first digit with preg_match
$phone = "Tel: 0555 123 45 67";
// Set the pattern and matches variable
$pattern = "/\d/";
preg_match($pattern, $phone, $matches);
// Print the first match
echo $matches[0] . "\n";
Test yourself
-
What does preg_match return?
- It returns the matched text itself
- It returns 1 if matched, 0 if not
- It always returns true
- It returns the index of the first match
Show answer
It returns 1 if matched, 0 if not — Correct. preg_match returns 1 on match, 0 on no match, false on error.
-
Add delimiters to the pattern
$pattern = "____";
echo preg_match($pattern, "apple pie") . "\n";
Show answer
$pattern = "/apple/";
echo preg_match($pattern, "apple pie") . "\n";
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Understand the regex pieces
- /.../
- Regex delimiters
- $matches
- Array holding the matches
- preg_match
- Function that finds the first match
- 1 or 0
- preg_match return value
Next lesson: Replacing with preg_replace()
