Match method and MatchData in Ruby - Step-by-Step Execution

Concept Flow - Match method and MatchData

Call str.match(pattern)

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Search for pattern in str

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Return MatchData

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Use MatchData to access captures or positions

The match method searches a string for a pattern. If found, it returns a MatchData object with details; if not, it returns nil.

Execution Sample

Ruby

text = "hello123"
result = text.match(/(\w+)(\d+)/)
puts result[0]
puts result[1]
puts result[2]

This code matches letters followed by digits in a string and prints the full match and captured groups.

Execution Table

Step	Action	Evaluation	Result
1	Call text.match(/(\w+)(\d+)/)	Search 'hello123' for pattern	Match found: 'hello123'
2	Access result[0]	Full matched string	'hello123'
3	Access result[1]	First capture group (\w+)	'hello'
4	Access result[2]	Second capture group (\d+)	'123'
5	If no match	Returns nil	nil (not reached here)

💡 Pattern found in string, so MatchData returned and used to access captures.

Variable Tracker

Variable	Start	After match	After accessing captures	Final
text	"hello123"	"hello123"	"hello123"	"hello123"
result	nil	MatchData object for 'hello123'	Same MatchData	Same MatchData

Key Moments - 2 Insights

Why does result[0] give the full matched string, not just the first capture?

Because in MatchData, index 0 always holds the entire matched substring, while indexes 1 and up hold capture groups (see execution_table rows 2 and 3).

What happens if the pattern is not found in the string?

The match method returns nil, so trying to access result[0] would cause an error. This is shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution_table, what does result[1] contain at step 3?

A The first capture group 'hello'

B The full matched string 'hello123'

C The second capture group '123'

D nil

💡 Hint

Check execution_table row 3 where result[1] is accessed.

At which step does the match method confirm no match was found?

A Step 3

B Step 5

C Step 1

D Step 2

💡 Hint

Look at execution_table row 5 describing no match case.

If the string was 'abc' and pattern /(\d+)/ was used, what would result be?

A MatchData with 'abc'

B MatchData with digits

C nil

D Error

💡 Hint

Refer to key_moments about no match returning nil.

Concept Snapshot

match(pattern) searches a string for a pattern.
Returns MatchData if found, else nil.
MatchData[0] is full match.
MatchData[1..] are capture groups.
Check for nil before accessing captures.

Full Transcript

The Ruby match method looks for a pattern inside a string. If it finds the pattern, it returns a MatchData object. This object holds the full matched text at index 0 and any captured groups at indexes 1 and beyond. If the pattern is not found, match returns nil. You can access parts of the match using result[0], result[1], etc. Always check if the result is nil before accessing captures to avoid errors.