Regular expressions in MATLAB - Mini Project: Build & Apply

Regular expressions in MATLAB

📖 Scenario: You work in a small company that processes customer feedback. The feedback contains many sentences, and you want to find specific words or patterns inside these sentences to analyze customer sentiment.

🎯 Goal: You will create a MATLAB script that uses regular expressions to find all words starting with the letter 'a' in a list of customer feedback sentences.

📋 What You'll Learn

Create a cell array of strings called feedback with exact sentences.

Create a pattern string variable called pattern to find words starting with 'a'.

Use the regexp function with the 'match' option to find all matching words.

Print the matched words using disp.

💡 Why This Matters

🌍 Real World

Regular expressions help find patterns in text data like customer feedback, emails, or logs.

💼 Career

Knowing how to use regular expressions in MATLAB is useful for data analysis, text processing, and automating report generation.

1

Create the feedback data

Create a cell array called feedback with these exact sentences: 'I absolutely love this product', 'Amazing service and friendly staff', 'The ambiance was average', 'Affordable prices and good quality'.

MATLAB

% Create the cell array feedback with the given sentences
% Your code here

Hint

Use curly braces {} to create a cell array of strings in MATLAB.

2

Define the regular expression pattern

Create a variable called pattern and set it to the regular expression string '\<[aA]\w*\>' to find words starting with the letter 'a'.

MATLAB

feedback = {"I absolutely love this product", "Amazing service and friendly staff", "The ambiance was average", "Affordable prices and good quality"};
% Define the pattern variable to find words starting with 'a'
% Your code here

Hint

The pattern '\<[aA]\w*\>' matches words starting with 'a'. Use double backslashes in MATLAB strings.

3

Find all matching words using regexp

Use the regexp function with variables feedback and pattern, and the option 'match' to find all words starting with 'a'. Store the result in a variable called matches.

MATLAB

feedback = {"I absolutely love this product", "Amazing service and friendly staff", "The ambiance was average", "Affordable prices and good quality"};
pattern = '\';
% Use regexp to find matches and store in matches
% Your code here

Hint

Use regexp with the 'match' option to get matching words as cell arrays.

4

Display the matched words

Use disp to print the variable matches to show all words starting with 'a' found in the feedback.

MATLAB

feedback = {"I absolutely love this product", "Amazing service and friendly staff", "The ambiance was average", "Affordable prices and good quality"};
pattern = '\';
matches = regexp(feedback, pattern, 'match');
% Display the matches variable
% Your code here

Hint

Use disp(matches) to print the cell array of matches.

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