Preg_match for pattern matching in PHP - Step-by-Step Execution

Concept Flow - Preg_match for pattern matching

Start with input string

↓

Define regex pattern

↓

Call preg_match(pattern, string)

↓

Check if pattern matches string?

No→Return 0 (no match)

Yes↓

Return 1 (match found)

↓

Optionally fill matches array

↓

End

The program takes a string and a regex pattern, uses preg_match to check if the pattern matches the string, returns 1 if yes, 0 if no, and optionally stores matched parts.

Execution Sample

PHP

<?php
$pattern = '/cat/';
$string = 'The cat sat.';
if (preg_match($pattern, $string, $matches)) {
    echo "Match found: " . $matches[0];
} else {
    echo "No match.";
}
?>

This code checks if 'cat' is in the string and prints the matched word if found.

Execution Table

Step	Action	Pattern	String	preg_match Result	Matches Array	Output
1	Define pattern	/cat/	N/A	N/A	N/A	N/A
2	Define string	N/A	The cat sat.	N/A	N/A	N/A
3	Call preg_match	/cat/	The cat sat.	1	["cat"]	N/A
4	Check result	N/A	N/A	1 (true)	N/A	N/A
5	Print match	N/A	N/A	N/A	N/A	Match found: cat

💡 preg_match returns 1 because 'cat' is found in the string.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
$pattern	undefined	/cat/	/cat/	/cat/	/cat/
$string	undefined	undefined	The cat sat.	The cat sat.	The cat sat.
$matches	undefined	undefined	undefined	["cat"]	["cat"]

Key Moments - 3 Insights

Why does preg_match return 1 instead of true or false?

preg_match returns 1 for a match, 0 for no match, and false on error. This is shown in execution_table row 3 where result is 1 meaning a match was found.

What is stored in the $matches array?

The $matches array stores the matched substring(s). In row 3, it contains ["cat"], the exact part of the string that matched the pattern.

What happens if the pattern is not found in the string?

preg_match returns 0 and $matches remains empty or unchanged. This is implied by the exit_note and would be shown if the string did not contain 'cat'.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $matches after step 3?

A []

B ["cat"]

C null

D ["dog"]

💡 Hint

Check the 'Matches Array' column in row 3 of the execution_table.

At which step does preg_match confirm a match was found?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'preg_match Result' and 'Action' columns in the execution_table.

If the string was 'The dog sat.', what would preg_match return?

A 0

B false

C 1

D null

💡 Hint

Refer to the exit_note and understand preg_match returns 0 when no match is found.

Concept Snapshot

preg_match(pattern, string, matches) checks if pattern matches string.
Returns 1 if match found, 0 if not, false on error.
Matches array stores matched parts.
Use /pattern/ syntax for regex.
Commonly used for simple pattern checks.

Full Transcript

This example shows how preg_match works in PHP. First, we define a regex pattern '/cat/'. Then we have a string 'The cat sat.'. Calling preg_match with these checks if 'cat' is inside the string. It returns 1 because it finds a match. The matched word 'cat' is stored in the matches array. If no match was found, preg_match would return 0. This helps us decide what to do next, like printing 'Match found: cat' or 'No match.'.