Lookahead and lookbehind in PHP - Step-by-Step Execution

Concept Flow - Lookahead and lookbehind

Start regex engine

↓

Check lookbehind condition

Yes↓

Match main pattern

↓

Check lookahead condition

Yes↓

Accept or reject match

The regex engine first checks the lookbehind condition before the main pattern, then matches the main pattern, and finally checks the lookahead condition after the main pattern to decide if the match is accepted.

Execution Sample

PHP

<?php
$pattern = '/(?<=\$)\d+(?=\sUSD)/';
$text = 'The price is $100 USD.';
preg_match($pattern, $text, $matches);
echo $matches[0];
?>

This code finds a number preceded by '$' and followed by ' USD' in the text.

Execution Table

Step	Action	Text Position	Condition Checked	Result	Match Found
1	Check lookbehind (?<=\$)	Before '1' in '100'	Is previous char '$'?	Yes	Continue
2	Match main pattern \d+	At '100'	Match digits	Matches '100'	Continue
3	Check lookahead (?=\sUSD)	After '100'	Is next chars ' USD'?	Yes	Accept match '100'
4	End	-	-	-	Match accepted: '100'

💡 Lookbehind and lookahead conditions met, so '100' is matched.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
matches	[]	[]	['100']	['100']	['100']

Key Moments - 2 Insights

Why does the regex check the lookbehind before matching digits?

Because lookbehind ensures the pattern before the main match is correct without consuming characters, as shown in step 1 of the execution_table.

What happens if the lookahead condition fails?

The match is rejected even if the main pattern matches, as the lookahead must confirm the following characters, shown in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is checked at Step 1?

A If the next characters are digits

B If the previous character is '$'

C If the main pattern matches digits

D If the following characters are ' USD'

💡 Hint

Check the 'Condition Checked' column at Step 1 in execution_table.

At which step does the regex confirm the main digits '100'?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Match Found' column to see when digits are matched.

If the text was 'The price is 100 USD.' (no '$'), what would happen at Step 1?

A Lookbehind check passes

B Main pattern matches anyway

C Lookbehind check fails, no match

D Lookahead check fails

💡 Hint

Look at Step 1 condition about previous character being '$' in execution_table.

Concept Snapshot

Lookbehind (?<=...) checks text before the match without consuming it.
Lookahead (?=...) checks text after the match without consuming it.
Both are zero-width assertions.
Used to match patterns only if preceded or followed by specific text.
In PHP, use preg_match with these in regex patterns.

Full Transcript

This example shows how PHP regex uses lookbehind and lookahead. The regex pattern '/(?<=\$)\d+(?=\sUSD)/' looks for digits preceded by a dollar sign and followed by a space and 'USD'. The engine first checks the lookbehind condition before matching digits, then matches the digits, and finally checks the lookahead condition. If all conditions pass, the match is accepted. Variables track the matched results. Common confusions include why lookbehind is checked first and what happens if lookahead fails. The visual quiz tests understanding of these steps.