Regular expressions with Regex class in Kotlin - Step-by-Step Execution

Concept Flow - Regular expressions with Regex class

Create Regex object

↓

Call match/search methods

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Check if pattern matches input

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Return match result or boolean

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Use result for logic or output

Create a Regex object, use it to check or find patterns in text, then use the results.

Execution Sample

Kotlin

val regex = Regex("a.b")
val input = "acb"
val match = regex.matches(input)
println(match)

Checks if the input string exactly matches the pattern 'a.b' and prints true or false.

Execution Table

Step	Action	Evaluation	Result
1	Create Regex object with pattern "a.b"	Regex("a.b")	Regex object created
2	Set input string	input = "acb"	input = "acb"
3	Call matches() to check full match	regex.matches("acb")	true (pattern matches 'acb')
4	Print match result	println(true)	Output: true
5	End of program	-	-

💡 Program ends after printing the match result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
regex	null	Regex("a.b")	Regex("a.b")	Regex("a.b")	Regex("a.b")
input	null	null	"acb"	"acb"	"acb"
match	null	null	null	true	true

Key Moments - 3 Insights

Why does regex.matches("acb") return true even though the pattern has a dot?

The dot in the pattern means 'any single character', so 'a.b' matches 'a' + any char + 'b'. Here, 'acb' fits exactly.

What if the input was "abc"? Would regex.matches("abc") be true?

No, because 'a.b' expects 'a' + any char + 'b'. 'abc' ends with 'c', so it does not match fully.

What does matches() check compared to containsMatchIn()?

matches() checks if the whole string fits the pattern exactly. containsMatchIn() checks if any part matches.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'match' after step 3?

A false

B null

C true

D "acb"

💡 Hint

Check the 'Result' column in row for step 3 in execution_table.

At which step is the input string assigned the value "acb"?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column for when input is set in execution_table.

If the input was changed to "abc", what would regex.matches(input) return?

A false

B true

C null

D Throws error

💡 Hint

Refer to key_moments explanation about pattern matching with 'a.b' and input 'abc'.

Concept Snapshot

Regex class in Kotlin:
- Create with Regex("pattern")
- Use matches() to check full string match
- Use containsMatchIn() to check partial match
- Dot (.) matches any single character
- matches() returns Boolean true/false

Full Transcript

This example shows how to use Kotlin's Regex class to check if a string matches a pattern. We create a Regex object with the pattern 'a.b', where dot means any character. Then we check if the input string 'acb' matches the pattern exactly using matches(). The result is true because 'acb' fits the pattern 'a' + any char + 'b'. We print the result. The execution table traces each step: creating Regex, setting input, matching, printing, and ending. Variables regex, input, and match change values as the program runs. Key moments clarify why the dot matches any character and the difference between matches() and containsMatchIn(). The quiz tests understanding of variable values and pattern matching behavior. The snapshot summarizes how to use Regex class simply.