Regular expressions in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of regular expressions in MATLAB?

Regular expressions in MATLAB are used to find, match, and manipulate text patterns within strings. They help automate searching and editing tasks.

beginner

Which MATLAB function is commonly used to find matches of a regular expression in a string?

The regexp function is used to find matches of a regular expression in a string and returns the starting indices of matches.

beginner

What does the regular expression pattern \d+ match in MATLAB?

The pattern \d+ matches one or more digits in a string. Here, \d means any digit (0-9), and + means one or more times.

intermediate

How do you extract the actual matched text using regexp in MATLAB?

Use the 'match' option with regexp like this: matches = regexp(str, pattern, 'match'); This returns the matched substrings themselves.

intermediate

What is the difference between regexp and regexpi in MATLAB?

regexp is case-sensitive when matching patterns, while regexpi performs case-insensitive matching.

Which MATLAB function returns the starting indices of matches for a regular expression?

A regexprep

B strfind

C regexp

D regexpi

regexp returns the starting indices of matches. regexprep replaces text, strfind finds substrings, and regexpi is case-insensitive regexp.

What does the regular expression ^abc match in a string?

A Any string containing 'abc'

B 'abc' at the start of the string

C 'abc' at the end of the string

D Any string ending with 'abc'

The caret ^ means the start of the string, so ^abc matches 'abc' only if it is at the beginning.

How do you perform a case-insensitive regular expression match in MATLAB?

A Use <code>regexpi</code>

B Use <code>strfind</code>

C Use <code>regexp</code> with 'ignorecase' option

D Use <code>regexprep</code>

regexpi performs case-insensitive matching directly.

What does the pattern \s represent in MATLAB regular expressions?

A Any digit

B Any special character

C Any letter

D Any whitespace character

\s matches any whitespace character like space, tab, or newline.

Which function would you use to replace text matching a regular expression in MATLAB?

A regexprep

B regexp

C regexpi

D strfind

regexprep replaces parts of a string that match a regular expression.

Explain how to find and extract all numbers from a string in MATLAB using regular expressions.

Think about how to match digits and get the actual text matched.

Describe the difference between regexp and regexpi in MATLAB and when you might use each.

Consider if you want to match text exactly or ignore letter case.

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