replace

Learn replacement with replace and lambda.

replace: swap matched parts

replace replaces every part that matches the pattern with a new string. The first argument is a Regex, the second is the replacement text.

val r = Regex("\\d+")
val result = "a1b22c".replace(r, "#")
// "a#b#c"

To swap digits with # you write the pattern once. replace scans every match in order and puts the new text in its place. Use replaceFirst to replace only the first match.

replace with a lambda: flexible transform

You can transform each match separately by passing a lambda. The lambda takes a MatchResult and returns the replacement string.

val r = Regex("\\w+")
val result = "hello world".replace(r) {
    it.value.replaceFirstChar { c -> c.uppercaseChar() }
}
// "Hello World"

To capitalise the first letter of each word, use replaceFirstChar. Pick the lambda form when you want a transform, not a fixed text. This lets you shape the whole string in one pass.

Exercise Replace digits with #

With Regex("\\d+"), replace the digits in "a1b22c333" with "#"; print the result.

Show example solution

val r = Regex("\\d+")
println("a1b22c333".replace(r, "#"))

Test yourself

Fill in the blank: replace digits

val r = Regex("\\d+")
val result = "a1b2".____(r, "#")

Show answer

val r = Regex("\\d+")
val result = "a1b2".replace(r, "#")

What does replace(r) { ... } do?
- Replace with fixed text
- Transform instead of fixed text
- Find the first match
- Count all matches
Show answer

Transform instead of fixed text — The lambda transforms each match into a different string.
Match method to outcome

replace(r, "#")

All with fixed
replace(r) { ... }

Transform per match
replaceFirst(r, "#")

Only the first
matches

Does all fit

Next lesson: StringBuilder

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