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.