Character classes and quantifiers in PHP - Step-by-Step Execution

Concept Flow - Character classes and quantifiers

Start

↓

Define regex pattern

↓

Input string

↓

Match regex

↓

Check character classes

↓

Apply quantifiers

↓

Return matches

↓

End

The program defines a regex pattern with character classes and quantifiers, then checks an input string for matches step-by-step.

Execution Sample

PHP

<?php
$pattern = '/[a-z]{2,4}\d+/';
$input = 'ab12 xyz123 abcdef456';
preg_match_all($pattern, $input, $matches);
print_r($matches[0]);
?>

This PHP code finds all substrings with 2 to 4 lowercase letters followed by one or more digits.

Execution Table

Step	Action	Pattern Part	Input Segment	Match Result	Output
1	Define pattern	[a-z]{2,4}\d+	-	Pattern ready	-
2	Input string	-	ab12 xyz123 abcdef456	Input ready	-
3	Match attempt 1	[a-z]{2,4}\d+	ab12	Matches 'ab12'	['ab12']
4	Match attempt 2	[a-z]{2,4}\d+	xyz123	Matches 'xyz123'	['ab12', 'xyz123']
5	Match attempt 3	[a-z]{2,4}\d+	abcdef456	No match (letters >4)	['ab12', 'xyz123']
6	End	-	-	Matching complete	['ab12', 'xyz123']

💡 No more matches found; input fully scanned.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
$pattern	/[a-z]{2,4}\d+/	/[a-z]{2,4}\d+/	/[a-z]{2,4}\d+/	/[a-z]{2,4}\d+/	/[a-z]{2,4}\d+/
$input	ab12 xyz123 abcdef456	ab12 xyz123 abcdef456	ab12 xyz123 abcdef456	ab12 xyz123 abcdef456	ab12 xyz123 abcdef456
$matches[0]	[]	['ab12']	['ab12', 'xyz123']	['ab12', 'xyz123']	['ab12', 'xyz123']

Key Moments - 2 Insights

Why does 'abcdef456' not match the pattern even though it has letters followed by digits?

Because the pattern requires 2 to 4 lowercase letters ([a-z]{2,4}) before digits, but 'abcdef' has 6 letters, exceeding the quantifier limit. See execution_table row 5.

What does the quantifier '+' after \d mean in the pattern?

It means one or more digits must follow the letters. So at least one digit is required for a match. This is why 'ab12' and 'xyz123' match. See execution_table rows 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $matches[0] after step 4?

A ['ab12']

B ['xyz123']

C ['ab12', 'xyz123']

D []

💡 Hint

Check the Output column at step 4 in execution_table.

At which step does the pattern fail to match due to too many letters?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at the Match Result column where it says 'No match (letters >4)'.

If the quantifier {2,4} was changed to {3,5}, which input segment would fail to match?

A 'ab12'

B 'xyz123'

C 'abcdef456'

D All would match

💡 Hint

Check variable_tracker for $matches and consider letter counts in input segments.

Concept Snapshot

Character classes: [a-z] matches any lowercase letter.
Quantifiers: {2,4} means match 2 to 4 times.
+ means one or more times.
Together: [a-z]{2,4}\d+ matches 2-4 letters followed by digits.
Used in PHP with preg_match_all to find matches in strings.

Full Transcript

This example shows how PHP uses regex with character classes and quantifiers. The pattern '/[a-z]{2,4}\d+/' looks for substrings with 2 to 4 lowercase letters followed by one or more digits. The input string 'ab12 xyz123 abcdef456' is scanned. Matches found are 'ab12' and 'xyz123'. 'abcdef456' does not match because it has 6 letters, exceeding the {2,4} limit. The execution table traces each matching attempt step-by-step, showing how variables change. Key points include understanding quantifiers and why some substrings fail to match. The visual quiz tests understanding of these steps and pattern behavior.