Regular expressions in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find matches of the pattern 'cat' in the text.

MATLAB

matches = regexp(text, '[1]');

Drag options to blanks, or click blank then click option'

A'dog'

B'cat'

C'bat'

D'rat'

Attempts:

3 left

💡 Hint

Common Mistakes

Using double quotes instead of single quotes for the pattern.

Using a pattern that does not exist in the text.

The pattern 'cat' correctly matches the substring 'cat' in the text.

2fill in blank

medium

Complete the code to extract all words starting with 'a' or 'A' from the text.

MATLAB

words = regexp(text, '[1]', 'match');

Drag options to blanks, or click blank then click option'

A'\b[aA]\w*\b'

B'\d+'

C'cat'

D'\s'

Attempts:

3 left

💡 Hint

Common Mistakes

Not using word boundaries, causing partial matches.

Using '\d+' which matches digits, not words.

The pattern '\b[aA]\w*\b' matches words starting with 'a' or 'A'.

3fill in blank

hard

Fix the error in the code to find the position of the first digit in the string.

MATLAB

pos = regexp(str, '[1]');

Drag options to blanks, or click blank then click option'

A'\D'

B'[a-zA-Z]'

C'\d'

D'\s'

Attempts:

3 left

💡 Hint

Common Mistakes

Using '[a-zA-Z]' which matches letters, not digits.

Using '\D' which matches non-digit characters.

The pattern '\d' matches any digit character, so it finds the first digit position.

4fill in blank

hard

Fill both blanks to extract all email addresses from the text.

MATLAB

emails = regexp(text, '[1]', '[2]');

Drag options to blanks, or click blank then click option'

A'[\w\.]+@[\w\.]+\.[a-zA-Z]{2,}'

B'start'

C'match'

D'end'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'end' instead of 'match' to get the actual emails.

Using an incomplete or incorrect email pattern.

The pattern matches typical email addresses, and 'match' returns the matched strings.

5fill in blank

hard

Fill all three blanks to replace all whitespace sequences with a single dash '-' in the string.

MATLAB

newStr = regexprep(str, '[1]', '[2]', '[3]');

Drag options to blanks, or click blank then click option'

A'\s+'

B'-'

C'all'

D'once'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'once' which replaces only the first match.

Using incorrect pattern that does not match whitespace sequences.

'\s+' matches one or more whitespace characters, '-' is the replacement, and 'all' replaces all occurrences.

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