Character classes and quantifiers in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of regex match with character classes

What will be the output of this PHP code snippet?

PHP

<?php
$pattern = '/[a-c]{2,4}/';
$subject = 'abcabcabc';
preg_match_all($pattern, $subject, $matches);
print_r($matches[0]);
?>

A Array ( [0] => abca [1] => bcab )

B Array ( [0] => ab [1] => ca [2] => bc )

C Array ( [0] => a [1] => b [2] => c )

D Array ( [0] => abc [1] => abc [2] => abc )

💡 Hint

Look at the pattern: it matches 2 to 4 characters from a to c.

The pattern '/[a-c]{2,4}/' matches sequences of 2 to 4 characters where each character is a, b, or c. In 'abcabcabc', the greedy quantifier takes the maximum of 4: 'abca' (positions 0-3), then 'bcab' (4-7).

❓ Predict Output

intermediate

Result of regex with negated character class

What will this PHP code output?

PHP

<?php
$pattern = '/[^0-9]{3}/';
$subject = 'abc123d4';
preg_match_all($pattern, $subject, $matches);
print_r($matches[0]);
?>

A Array ( [0] => a1b [1] => 2c3 [2] => d4 )

B Array ( [0] => b2c )

C Array ( [0] => bcd )

D Array ( [0] => abc )

💡 Hint

The pattern matches 3 characters that are NOT digits.

The pattern '/[^0-9]{3}/' matches exactly 3 consecutive non-digit characters. In 'abc123d4', 'abc' matches but 'd' is only 1 character.

🔧 Debug

advanced

Identify the error in regex quantifier usage

This PHP code throws an error. What is the cause?

PHP

<?php
$pattern = '/[a-z]{3,2}/';
$subject = 'abcdef';
preg_match($pattern, $subject, $matches);
print_r($matches);
?>

A RuntimeError: preg_match failed due to invalid pattern

B No error, matches 'abc'

C SyntaxError: Quantifier range is invalid because 3 is greater than 2

D TypeError: preg_match expects string, got array

💡 Hint

Check the quantifier {3,2} carefully.

Quantifiers in regex must have the lower number first. '{3,2}' is invalid because 3 is greater than 2. This causes a regex compilation error.

📝 Syntax

advanced

Which regex pattern matches exactly 1 or 2 digits?

Choose the correct regex pattern that matches exactly one or two digits in PHP.

A '/\d{1,2}/'

B '/\d{1,}/'

C '/\d{,2}/'

D '/\d{2,1}/'

💡 Hint

Quantifiers specify minimum and maximum counts in the form {min,max}.

Option A uses '{1,2}' which means match 1 or 2 digits. Option A means 1 or more digits, option A is invalid syntax, and option A has min greater than max which is invalid.

🚀 Application

expert

Count words with 3 to 5 letters using regex

You want to count how many words in a string have between 3 and 5 letters (only letters a-z, case insensitive). Which PHP code snippet correctly does this?

A

<?php
preg_match_all('/\b[a-zA-Z]{3,5}/', $text, $matches);
echo count($matches[0]);
?>

B

<?php
preg_match_all('/\b[a-z]{3,5}\b/i', $text, $matches);
echo count($matches[0]);
?>

C

<?php
preg_match_all('/\b[a-zA-Z]{3,5}\b/', $text, $matches);
echo count($matches[0]);
?>

D

<?php
preg_match_all('/[a-zA-Z]{3,5}\b/', $text, $matches);
echo count($matches[0]);
?>

💡 Hint

Use word boundaries and case-insensitive flag to match words correctly.

Option B uses word boundaries '\b' to ensure whole words, the character class '[a-z]' with 'i' flag for case insensitivity, and quantifier '{3,5}' to match words with 3 to 5 letters. Other options miss case insensitivity or word boundaries on both sides.