Regular expressions in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of regexpi with case-insensitive matching

What is the output of the following MATLAB code?

str = 'Matlab is fun';
expr = 'MAT';
idx = regexpi(str, expr);
disp(idx);

MATLAB

str = 'Matlab is fun';
expr = 'MAT';
idx = regexpi(str, expr);
disp(idx);

A []

B [1]

C [4]

D [7]

💡 Hint

regexpi is case-insensitive and returns start indices of matches.

The expression 'MAT' matches 'Mat' at the start of the string ignoring case, so the index is 1.

❓ Predict Output

intermediate

Using regexp to extract tokens

What does the following MATLAB code output?

str = 'abc123def456';
tokens = regexp(str, '\\d+', 'match');
disp(tokens);

MATLAB

str = 'abc123def456';
tokens = regexp(str, '\d+', 'match');
disp(tokens);

A {'abc', 'def'}

B {'1', '2', '3', '4', '5', '6'}

C {'123', '456'}

D []

💡 Hint

The pattern '\d+' matches one or more digits.

regexp with 'match' returns all substrings matching the pattern '\d+', which are '123' and '456'.

❓ Predict Output

advanced

Output of regexprep with backreference

What is the output of this MATLAB code?

str = 'cat bat rat';
newStr = regexprep(str, '(\w+)at', '$1ot');
disp(newStr);

MATLAB

str = 'cat bat rat';
newStr = regexprep(str, '(\w+)at', '$1ot');
disp(newStr);

A 'cat bot rat'

B 'cat bat rat'

C 'cot bat rat'

D 'cot bot rot'

💡 Hint

Backreference $1 refers to the first captured group.

The pattern captures the first letter before 'at' and replaces 'at' with 'ot', changing 'cat' to 'cot', 'bat' to 'bot', and 'rat' to 'rot'.

❓ Predict Output

advanced

Error type from invalid regex pattern

What error does this MATLAB code produce?

str = 'hello';
expr = '(hello';
matches = regexp(str, expr);

MATLAB

str = 'hello';
expr = '(hello';
matches = regexp(str, expr);

A Error: Unbalanced or unexpected parenthesis.

B Error: Undefined function or variable 'hello'.

C Error: Invalid character in expression.

D No error, matches = [1].

💡 Hint

Check if the parentheses in the pattern are balanced.

The pattern '(hello' has an opening parenthesis without a closing one, causing an unbalanced parenthesis error.

🧠 Conceptual

expert

Number of matches found by regexpi with overlapping patterns

Consider the MATLAB code below:

str = 'aaaa';
expr = 'aa';
matches = regexpi(str, expr);
numMatches = length(matches);

What is the value of numMatches after running this code?

MATLAB

str = 'aaaa';
expr = 'aa';
matches = regexpi(str, expr);
numMatches = length(matches);

A 2

B 3

C 4

D 1

💡 Hint

regexpi does not find overlapping matches by default.

regexpi finds 'aa' starting at positions 1 and 3, but not overlapping at position 2, so total matches are 2.

Practice

1. What does the MATLAB function regexp do?

easy

A. Searches for text patterns in strings

B. Replaces text in strings

C. Sorts arrays numerically

D. Calculates the length of a string

Solution

1. Step 1: Understand the purpose of regexp

   The regexp function in MATLAB is used to find matches of a pattern within text.

2. Step 2: Compare with other functions

   regexprep replaces text, but regexp only searches for patterns.

3. Final Answer:

   Searches for text patterns in strings -> Option A

4. Quick Check:

   regexp = search function [OK]

Hint: Remember: regexp = search, regexprep = replace [OK]

Common Mistakes:

• Confusing regexp with regexprep
• Thinking regexp modifies strings
• Mixing regexp with numeric functions

2. Which of the following is the correct syntax to find all digits in the string 'abc123' using regexp?

easy

A. regexp('abc123', '\\d+')

B. regexp('abc123', '\\w+')

C. regexp('abc123', '\\s+')

D. regexp('abc123', '\\D+')

Solution

1. Step 1: Identify the pattern for digits

   In regular expressions, \d matches digits, and + means one or more.

2. Step 2: Check the options

   regexp('abc123', '\\d+') uses \\d+ which correctly matches one or more digits. Others match word characters, spaces, or non-digits.

3. Final Answer:

   regexp('abc123', '\\d+') -> Option A

4. Quick Check:

   Digits = \d+ pattern [OK]

Hint: Use \\d+ to match one or more digits [OK]

Common Mistakes:

• Using \w+ which matches letters and digits
• Using \s+ which matches spaces
• Using \D+ which matches non-digits

3. What is the output of the following MATLAB code?

str = 'cat bat rat';
matches = regexp(str, '\\b\w{3}\b', 'match');
disp(matches);

medium

A. ['cat', 'bat']

B. ['catbat', 'rat']

C. [] (empty cell array)

D. ['cat', 'bat', 'rat']

Solution

1. Step 1: Understand the pattern '\\b\\w{3}\\b'

   The pattern matches word boundaries \\b around exactly 3 word characters \\w{3}.

2. Step 2: Apply pattern to string

   The string has three 3-letter words: 'cat', 'bat', and 'rat'. All match the pattern.

3. Final Answer:

   ['cat', 'bat', 'rat'] -> Option D

4. Quick Check:

   Three 3-letter words matched [OK]

Hint: \b\w{3}\b matches exactly 3-letter words [OK]

Common Mistakes:

• Ignoring word boundaries and matching partial words
• Expecting concatenated matches
• Confusing \w with digits only

4. The following MATLAB code is intended to replace all digits in str with '#' but does not work as expected:

str = 'Room 101';
newStr = regexprep(str, '\d', '#');
disp(newStr);

What is the error and how to fix it?

medium

A. Pattern should be '\\d+' to replace all digits at once

B. No error; code works correctly

C. Pattern should be '\\D' to match digits

D. Use regexp instead of regexprep

Solution

1. Step 1: Analyze the regexprep usage

   The pattern '\\d' matches any single digit, and regexprep replaces all occurrences by default.

2. Step 2: Check the output

   Replacing each digit with '#' in 'Room 101' results in 'Room ###', which is expected.

3. Final Answer:

   No error; code works correctly -> Option B

4. Quick Check:

   regexprep replaces all matches by default [OK]

Hint: regexprep replaces all matches even with single-digit pattern [OK]

Common Mistakes:

• Thinking \d replaces only one digit total
• Using \D which matches non-digits
• Confusing regexp with regexprep

5. You have a cell array of strings: {'abc123', 'def456', 'ghi789'}. How do you extract only the numeric parts from each string using MATLAB regular expressions?

hard

A. Use regexp(cellArray, '\\d+', 'match')

B. Use regexp(cellArray, '\\w+', 'match')

C. Use regexprep(cellArray, '\\D', '')

D. Use regexprep(cellArray, '\\d+', '')

Solution

1. Step 1: Understand the goal

   We want to keep only digits, removing all non-digit characters.

2. Step 2: Choose the correct function and pattern

   regexprep replaces patterns. Using \\D matches non-digits, so replacing them with empty string keeps only digits.

3. Final Answer:

   Use regexprep(cellArray, '\\D', '') -> Option C

4. Quick Check:

   Remove non-digits with \D pattern [OK]

Hint: Replace non-digits (\D) with empty string to keep digits [OK]

Common Mistakes:

• Using regexp with 'match' returns cell arrays, not strings
• Replacing digits instead of non-digits
• Using \w+ which matches letters and digits