Regex

Learn regular expressions with Regex.

Regex: matching with patterns

In Kotlin the Regex class matches text against a pattern. A pattern is a string with special symbols: \d digit, \w letter digit underscore, + one or more, * zero or more.

val digits = Regex("\\d+")
val result = digits.matches("12345")
// true

Inside a normal string you must double the backslash. With a raw string a single backslash is enough. Regex is ideal for validating phone, email or date-like strings.

Using the Regex class

A Regex object is created with Regex(pattern). If you use the same pattern many times, store it in a variable so it is not recompiled.

val phone = Regex("\\d{3}-\\d{4}")
println(phone.matches("555-1234"))
// true

In a pattern {n} means exactly n times, {n,m} means at least n at most m. This lets you write fixed-length patterns easily. Define the pattern once, then match as many times as you need.

Exercise Test with a Regex

Define a Regex that matches one or more digits. Test the string "2024" with matches and print the result.

Show example solution

val r = Regex("\\d+")
println(r.matches("2024"))

Test yourself

Match pattern to meaning

\d

Digit
\w

Letter digit underscore
+

One or more
*

Zero or more
Fill in the blank: define a Regex
```
val r = ____("\\d+")
```
Show answer
```
val r = Regex("\\d+")
```
Which pattern captures the digits in 'abc123'?
```
val r = Regex("\\d+")
```
- \d?
- \w-
- [abc]+
- \d+
Show answer

\d+ — \d+ matches one or more digits.

Next lesson: matches and find

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies