Regular expressions in MATLAB - Step-by-Step Execution

Concept Flow - Regular expressions in MATLAB

Input String

↓

Define Pattern

↓

Use regexp Function

↓

Match Found?

No→Return Empty

Yes↓

Return Match Indices or Tokens

↓

Use Matches for Further Processing

The flow shows how MATLAB takes an input string, applies a regular expression pattern using regexp, checks for matches, and returns the results for further use.

Execution Sample

MATLAB

str = 'cat bat rat';
pattern = '\w+at';
matches = regexp(str, pattern, 'match');
disp(matches);

This code finds all words ending with 'at' in the string and displays them.

Execution Table

Step	Action	Input	Pattern	Result	Output
1	Define input string	'cat bat rat'
2	Define pattern		\w+at
3	Call regexp	'cat bat rat'	\w+at	Find all matches of words ending with 'at'
4	Match 'cat'			Match found at indices 1-3	matches = {'cat'}
5	Match 'bat'			Match found at indices 5-7	matches = {'cat', 'bat'}
6	Match 'rat'			Match found at indices 9-11	matches = {'cat', 'bat', 'rat'}
7	Return matches				Display {'cat', 'bat', 'rat'}

💡 All matches found; regexp returns all words ending with 'at' in the string.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	Final
str	''	'cat bat rat'	'cat bat rat'	'cat bat rat'	'cat bat rat'
pattern	''	'\w+at'	'\w+at'	'\w+at'	'\w+at'
matches	{}	{'cat'}	{'cat', 'bat'}	{'cat', 'bat', 'rat'}	{'cat', 'bat', 'rat'}

Key Moments - 3 Insights

Why does the pattern use double backslashes like '\\w+at' instead of '\w+at'?

In MATLAB strings, a single backslash is an escape character, so to represent a literal backslash in the pattern, you need to use two backslashes. This is shown in the execution_table where the pattern is '\\w+at'.

What does the 'match' option in regexp do?

The 'match' option tells regexp to return the actual matching substrings instead of their indices. This is why the output in the execution_table shows the matched words like 'cat', 'bat', and 'rat'.

What happens if no matches are found?

If no matches are found, regexp returns an empty cell array. This is shown in the concept_flow where the 'Match Found?' decision leads to returning empty if no matches exist.

Visual Quiz - 3 Questions

Look at the execution_table at Step 5. What is the content of 'matches' after this step?

A {'cat'}

B {'cat', 'bat'}

C {'bat'}

D {}

💡 Hint

Check the 'Output' column in execution_table row for Step 5.

At which step does the regexp function find the match 'rat'?

A Step 6

B Step 5

C Step 4

D Step 7

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for the step matching 'rat'.

If the input string was 'dog log fog', what would the 'matches' variable contain after running the same code?

A {'dog', 'log', 'fog'}

B {'og'}

C {}

D {'dog'}

💡 Hint

Refer to the pattern '\\w+at' and check if any words in 'dog log fog' end with 'at'.

Concept Snapshot

MATLAB regexp usage:
- Define input string and pattern (use double backslashes for escapes)
- Call regexp(str, pattern, 'match') to get matching substrings
- Returns cell array of matches or empty if none
- Useful for finding patterns like words ending with 'at'
- Matches can be processed further or displayed

Full Transcript

This visual execution shows how MATLAB uses regular expressions with the regexp function. First, an input string is defined, then a pattern is set using double backslashes to escape special characters. The regexp function is called with the 'match' option to find all substrings matching the pattern. Each match is found step-by-step, updating the matches variable. If no matches are found, an empty cell array is returned. This process helps extract parts of strings that fit a pattern, like words ending with 'at'.