How to use Kotlin Regex

The Kotlin Regex class can be used to create a regular expression pattern that helps you find or replace matching strings.

The syntax of the Regex class is as follows:

Regex(pattern: String, option: RegexOption)

// or pass multiple options as a Set

Regex(pattern: String, option: Set<RegexOption>)

Let’s see an example of Regex in action.

To create a regular expression pattern, create a new instance of the Regex class as follows:

val regex = Regex("Island")
val text = "The Great Britain Island is located east of the Island of Ireland"
val matches = regex.findAll(text)

matches.forEach {
    println(it.value)
}

// result:
// Island
// Island

The instance of the Regex class can then be used to search or replace any string text by calling a specific method from the instance.

In the example above, the findAll() method is called to search for the Island string from the text variable value.

You can also pass a second argument to the Regex call as follows:

The list of methods that you can call from the Regex object can be found in the Kotlin documentation.

Some of the most useful methods are as follows:

• find()
• findAll()
• matchAt()
• matchEntire()
• replace()
• replaceFirst()

Kotlin also has the RegexOption enum class that’s used to set the options for the running method.

The list of available options are as follows:

• IGNORE_CASE
• MULTILINE
• LITERAL
• UNIX_LINES
• COMMENTS
• DOT_MATCHES_ALL
• CANON_EQ

For example, the following code replaces the word Island with Isle while ignoring the cases:

val regex = Regex("island", RegexOption.IGNORE_CASE)
val text = "The Great Britain Island is located east of the Island of Ireland"

val newText = regex.replace(text, "isle")
println(newText)

// output:
// The Great Britain isle is located east of the isle of Ireland

You can create any kind of valid regular expression to use with the Kotlin Regex class.