Capturing groups and backreferences in PHP - Step-by-Step Execution

Concept Flow - Capturing groups and backreferences

Start regex

↓

Find ( ) capturing group

↓

Match pattern inside group

↓

Store matched text

↓

Use backreference \1 to match same text again

↓

Full pattern matched?

No→Fail

Yes↓

Return match with captured groups

The regex engine finds text matching the pattern inside parentheses, stores it, then uses \1 to match the same text again.

Execution Sample

PHP

<?php
$pattern = '/(\w+) \1/';
$subject = 'hello hello world';
preg_match($pattern, $subject, $matches);
print_r($matches);
?>

This code finds repeated words like 'hello hello' using a capturing group and backreference.

Execution Table

Step	Regex Position	Action	Matched Text	Captured Group 1	Backreference \1 Match	Result
1	Start	Begin matching at start of subject				Continue
2	(\w+)	Match first word characters	hello	hello		Continue
3		Match space character		hello		Continue
4	\1	Match same text as group 1	hello	hello	hello	Continue
5	End	Pattern fully matched	hello hello	hello	hello	Match found
6	After match	Stop searching		hello	hello	Exit

💡 Pattern matched 'hello hello' and stopped searching further.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
Captured Group 1		hello	hello	hello
Backreference \1			hello	hello

Key Moments - 2 Insights

Why does the backreference \1 match the same text as the first group?

Because \1 refers exactly to the text captured by the first parentheses group, as shown in execution_table step 4.

What happens if the backreference \1 does not find the same text?

The pattern fails to match and the regex engine stops, as no full match is found (not shown in this successful trace).

Visual Quiz - 3 Questions

Look at the execution table, what text is captured by group 1 at step 2?

A hello

B world

C hello hello

D \1

💡 Hint

Check the 'Captured Group 1' column at step 2 in the execution_table.

At which step does the backreference \1 successfully match the same text?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Backreference \1 Match' column in the execution_table.

If the subject was 'hello world', what would happen to the match?

A Match found with 'hello world'

B No match because backreference \1 fails

C Match found with 'world world'

D Match found with 'hello hello'

💡 Hint

Backreference \1 must match the exact text captured by group 1; see key_moments about failure.

Concept Snapshot

Capturing groups use parentheses ( ) to save matched text.
Backreferences like \1 match the exact text from the first group.
Used to find repeated patterns, e.g., repeated words.
If backreference fails, the whole match fails.
Syntax example: /(\w+) \1/ matches repeated words.

Full Transcript

This example shows how PHP regex uses capturing groups and backreferences. The pattern /(\w+) \1/ looks for a word, then the same word again separated by space. Step by step, the regex matches 'hello' as group 1, then matches a space, then uses \1 to match the same 'hello' again. The match succeeds with 'hello hello'. Variables track the captured group and backreference text. If the backreference does not match, the pattern fails. This helps find repeated text patterns easily.