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.

Practice

1. Which character class in PHP regex matches any digit from 0 to 9?
/[?]/

easy

A. \\D

B. \\w

C. \\s

D. \\d

Solution

1. Step 1: Understand character classes

   In PHP regex, \\d matches any digit (0-9), \\w matches word characters, \\s matches whitespace, and \\D matches non-digits.

2. Step 2: Identify the digit matcher

   The class that matches digits is \\d.

3. Final Answer:

   \\d -> Option D

4. Quick Check:

   Digits = \\d [OK]

Hint: Remember: \d = digit, \w = word, \s = space [OK]

Common Mistakes:

• Confusing \d with \w
• Using \D to match digits
• Mixing up \s with digit class

2. Which of the following is the correct syntax to match exactly three lowercase letters using PHP regex?

easy

A. /[a-z]{3}/

B. /[a-z]{,3}/

C. /[a-z]{3,}/

D. /[a-z]{3,5}/

Solution

1. Step 1: Understand quantifiers

   Curly braces {n} specify exact count. {3} means exactly 3 times.

2. Step 2: Match exactly three lowercase letters

   The pattern /[a-z]{3}/ matches exactly three lowercase letters.

3. Final Answer:

   /[a-z]{3}/ -> Option A

4. Quick Check:

   Exact 3 lowercase letters = /[a-z]{3}/ [OK]

Hint: Use {n} for exact count in regex quantifiers [OK]

Common Mistakes:

• Using {,3} which is invalid syntax
• Using {3,} which means 3 or more
• Using {3,5} which means 3 to 5 times

3. What will be the output of this PHP code?

$pattern = '/\d{2,4}/';
$string = 'Year 2023 and 99 problems';
preg_match_all($pattern, $string, $matches);
print_r($matches[0]);

medium

A. ["20", "23", "99"]

B. ["2023", "99"]

C. ["202", "3", "99"]

D. ["2", "0", "2", "3", "9", "9"]

Solution

1. Step 1: Understand the regex pattern

   The pattern /\d{2,4}/ matches sequences of digits between 2 and 4 digits long.

2. Step 2: Apply pattern to string

   In 'Year 2023 and 99 problems', '2023' (4 digits) and '99' (2 digits) match. Smaller digit groups like '2' or '3' alone do not match.

3. Final Answer:

   ["2023", "99"] -> Option B

4. Quick Check:

   Digits 2-4 long = ["2023", "99"] [OK]

Hint: Quantifier {2,4} matches digit groups length 2 to 4 [OK]

Common Mistakes:

• Splitting digits into smaller groups
• Ignoring quantifier range
• Assuming single digits match

4. Identify the error in this PHP regex pattern:

$pattern = '/[a-z]{2,1}/';

medium

A. Quantifier range is invalid; min cannot be greater than max

B. Character class brackets are missing

C. Pattern should use double quotes instead of single quotes

D. No error; pattern is valid

Solution

1. Step 1: Analyze quantifier syntax

   The quantifier {2,1} means minimum 2 and maximum 1 repetitions, which is invalid because min > max.

2. Step 2: Confirm error type

   Regex quantifiers must have min ≤ max. This causes a syntax error.

3. Final Answer:

   Quantifier range is invalid; min cannot be greater than max -> Option A

4. Quick Check:

   Min > max in {2,1} is invalid [OK]

Hint: Quantifier {min,max} must have min ≤ max [OK]

Common Mistakes:

• Swapping min and max values
• Ignoring quantifier syntax rules
• Assuming pattern is valid without checking quantifiers

5. You want to match a string that contains exactly 3 to 5 uppercase letters followed by 2 digits in PHP regex. Which pattern is correct?

hard

A. /^[A-Z]{3,5}\d{1,2}$/

B. /^[A-Z]{2,3}\d{5}$/

C. /^[A-Z]{3,5}\d{2}$/

D. /^[A-Z]{3,5}\d{3}$/

Solution

1. Step 1: Understand the requirements

   The string must start (^) with 3 to 5 uppercase letters [A-Z]{3,5} and end ($) with exactly 2 digits \d{2}.

2. Step 2: Match pattern to options

   /^[A-Z]{3,5}\d{2}$/ matches exactly 3-5 uppercase letters followed by exactly 2 digits, anchored start and end.

3. Final Answer:

   /^[A-Z]{3,5}\d{2}$/ -> Option C

4. Quick Check:

   3-5 uppercase + 2 digits = /^[A-Z]{3,5}\d{2}$/ [OK]

Hint: Use anchors ^ and $ to match whole string exactly [OK]

Common Mistakes:

• Mixing digit counts in quantifiers
• Omitting anchors causing partial matches
• Using wrong quantifier ranges