Kotlin regular expressions
Kotlin regular expressions tutorial shows how to use regular expressions in Kotlin. The examples work with quantifiers, character classes, alternations, and groups.
Kotlin regular expressions
last modified January 29, 2024
This article shows how to use regular expressions in Kotlin.
Regular expressions are used for text searching and more advanced text manipulation. Regular expressions are built into tools such as grep, sed, text editors such as vi, Emacs, programming languages including Kotlin, JavaScript, Perl, and Python.
Kotlin regular expression
In Kotlin, we build regular expressions with the Regex.
Regex(“book”) “book”.toRegex() Regex.fromLiteral(“book”)
A pattern is a regular expression that defines the text we are searching for or manipulating. It consists of text literals and metacharacters. Metacharacters are special characters that control how the regular expression is going to be evaluated. For instance, with \s we search for white spaces.
Special characters must be double escaped or we can use Kotlin raw strings.
After we have created a pattern, we can use one of the functions to apply the pattern on a text string. The funcions include matches, containsMatchIn, find, findall, replace, and split.
The following table shows some commonly used regular expressions:
Regex
Meaning
.
Matches any single character.
?
Matches the preceding element once or not at all.
+
Matches the preceding element once or more times.
*
Matches the preceding element zero or more times.
^
Matches the starting position within the string.
$
Matches the ending position within the string.
|
Alternation operator.
[abc]
Matches a or b, or c.
[a-c]
Range; matches a or b, or c.
[^abc]
Negation, matches everything except a, or b, or c.
\s
Matches white space character.
\w
Matches a word character; equivalent to [a-zA-Z_0-9]
Kotlin matches and containsMatchIn methods
The matches method returns true if the regular expression matches the entire input string. The containsMatchIn method indicates whether the regular expression can find at least one match in the specified input.
simple_regex.kt
package com.zetcode
fun main() {
val words = listOf("book", "bookworm", "Bible",
"bookish","cookbook", "bookstore", "pocketbook")
val pattern = "book".toRegex()
println("*********************")
println("containsMatchIn function")
words.forEach { word ->
if (pattern.containsMatchIn(word)) {
println("$word matches")
}
}
println("*********************")
println("matches function")
words.forEach { word ->
if (pattern.matches(word)) {
println("$word matches")
}
}
}
In the example, we use the matches and containsMatchIn methods. We have a list of words. The pattern will look for a ‘book’ string in each of the words using both methods.
val pattern = “book”.toRegex()
A regular expression pattern is created with toRegex method. The regular expression consists of four normal characters.
words.forEach { word -> if (pattern.containsMatchIn(word)) { println("$word matches") } }
We iterate over the list and apply containsMatchIn on each of the words.
words.forEach { word -> if (pattern.matches(word)) { println("$word matches") } }
We iterate over the list again and apply matches on each of the words.
containsMatchIn function book matches bookworm matches bookish matches cookbook matches bookstore matches pocketbook matches
matches function book matches
For the containsMatchIn method, the pattern matches if the ‘book’ word is somewhere in the word; for the matches, the input string must entirely match the pattern.
Kotlin find method
The find method returns the first match of a regular expression in the input, beginning at the specified start index. The start index is 0 by default.
regex_find.kt
package com.zetcode
fun main() {
val text = "I saw a fox in the wood. The fox had red fur."
val pattern = "fox".toRegex()
val match = pattern.find(text)
val m = match?.value
val idx = match?.range
println("$m found at indexes: $idx")
val match2 = pattern.find(text, 11)
val m2 = match2?.value
val idx2 = match2?.range
println("$m2 found at indexes: $idx2")
}
In the example, we find out the indexes of the match of the ‘fox’ term.
val match = pattern.find(text)
val m = match?.value val idx = match?.range
We find the first match of the ‘fox’ term. We get its value and indexes.
val match2 = pattern.find(text, 11)
val m2 = match2?.value val idx2 = match2?.range
In the second case, we start the search from the index 11, finding thus the next term.
fox found at indexes: 8..10 fox found at indexes: 29..31
Kotlin findAll method
The findAll method returns a sequence of all occurrences of a regular expression within the input string.
regex_findall.kt
package com.zetcode
fun main() {
val text = "I saw a fox in the wood. The fox had red fur."
val pattern = "fox".toRegex()
val matches = pattern.findAll(text)
matches.forEach { f ->
val m = f.value
val idx = f.range
println("$m found at indexes: $idx")
}
}
In the example, we find all occurrences of the ‘fox’ term with findAll.
Kotlin regex word boundaries
The metacharacter \b is an anchor which matches at a position that is called a word boundary. It allows to search for whole words.
word_boundaries.kt
package com.zetcode
fun main() {
val text = "This island is beautiful"
val pattern = "\\bis\\b".toRegex()
val matches = pattern.findAll(text)
matches.forEach { m ->
val v = m.value
val idx = m.range
println("$v found at indexes: $idx")
}
}
In the example, we look for the is word. We do not want to include the This and the island words.
val pattern = “\bis\b”.toRegex()
With two \b metacharacters, we search for the is whole word.
val matches = pattern.findAll(text)
With the findAll function, we find all matches.
is found at indexes: 12..13
Kotlin regex implicit word boundaries
The \w is a character class used for a character allowed in a word. For the \w+ regular expression, which denotes a word, the leading and trailing word boundary metacharacters are implicit; i.e. \w+ is equal to \b\w+\b.
implicit_word_boundaries.kt
package com.zetcode
fun main() { val content = """ Foxes are omnivorous mammals belonging to several genera of the family Canidae. Foxes have a flattened skull, upright triangular ears, a pointed, slightly upturned snout, and a long bushy tail. Foxes live on every continent except Antarctica. By far the most common and widespread species of fox is the red fox."""
val pattern = "\\w+".toRegex()
val words = pattern.findAll(content)
val count = words.count()
println("There are $count words")
words.forEach { matchResult ->
println(matchResult.value)
}
}
In the example, we search for all words in the text.
val pattern = “\w+".toRegex()
We look for words.
val words = pattern.findAll(content) val count = words.count()
We find all the words and count them.
Kotlin currency symbols
The \p{Sc} regular expresion can be used to look for currency symbols.
currency_symbols.kt
package com.zetcode
fun main() {
val content = """
Currency symbols: ฿ Thailand bath, ₹ Indian rupee, ₾ Georgian lari, $ Dollar, € Euro, ¥ Yen, £ Pound Sterling”""
val pattern = "\\p{Sc}".toRegex(RegexOption.IGNORE_CASE)
val matches = pattern.findAll(content)
matches.forEach { matchResult ->
val currency = matchResult.value
val idx = matchResult.range
println("$currency at $idx")
}
}
In the example, we look for currency symbols.
val content = """
Currency symbols: ฿ Thailand bath, ₹ Indian rupee, ₾ Georgian lari, $ Dollar, € Euro, ¥ Yen, £ Pound Sterling"""
We have a couple of currency symbols in the text.
val pattern = “\p{Sc}".toRegex(RegexOption.IGNORE_CASE)
We define the regular expression for the currency symbols.
val matches = pattern.findAll(content)
We find all the matches.
matches.forEach { matchResult ->
val currency = matchResult.value
val idx = matchResult.range
println("$currency at $idx")
}
We print all the matched values and their indexes.
฿ at 19..19 ₹ at 36..36 ₾ at 52..52 $ at 69..69 € at 79..79 ¥ at 87..87 £ at 94..94
Kotlin split function
The split method splits the input string around matches of the regular expression.
regex_split.js
package com.zetcode
fun main() {
val text = "I saw a fox in the wood. The fox had red fur."
val pattern = "\\W+".toRegex()
val words = pattern.split(text).filter { it.isNotBlank() }
println(words)
}
In the exmaple, we find out the number of occurrences of the ‘fox’ term.
val pattern = “\W+".toRegex()
The pattern contains the \W named character class, which stands for non-word character. In conjunction with the + quantifier, the pattern looks for non-word character(s) such as space, comma, or dot, which are often used to separate words in text. Note that the character class is double escaped.
val words = pattern.split(text).filter { it.isNotBlank() }
With the split method, we split the input string into a list of words. In addition, we remove the blank trailing word, which was created because our text ended in a non-word character.
[I, saw, a, fox, in, the, wood, The, fox, had, red, fur]
Case insensitive match
To enable case insensitive search, we pass the RegexOption.IGNORE_CASE to the toRegex method.
regex_case_insensitive.kt
package com.zetcode
fun main() {
val words = listOf("dog", "Dog", "DOG", "Doggy")
val pattern = "dog".toRegex(RegexOption.IGNORE_CASE)
words.forEach { word ->
if (pattern.matches(word)) {
println("$word matches")
}
}
}
In the example, we apply the pattern on words regardless of the case.
val pattern = “dog”.toRegex(RegexOption.IGNORE_CASE)
We use the RegexOption.IGNORE_CASE to ignore the case of the input string.
dog matches Dog matches DOG matches
The dot metacharacter
The dot (.) metacharacter stands for any single character in the text.
regex_dot_meta.kt
package com.zetcode
fun main() {
val words = listOf("seven", "even", "prevent", "revenge", "maven",
"eleven", "amen", "event")
val pattern = "..even".toRegex()
words.forEach { word ->
if (pattern.containsMatchIn(word)) {
println("$word matches")
}
}
}
In the example, we have eight words in a list. We apply a pattern containing two dot metacharacters on each of the words.
prevent matches eleven matches
There are two words that match the pattern.
Question mark meta character
The question mark (?) meta character is a quantifier that matches the previous element zero or one time.
regex_qmark_meta.kt
package com.zetcode
fun main() {
val words = listOf("seven", "even", "prevent", "revenge", "maven",
"eleven", "amen", "event")
val pattern = ".?even".toRegex()
words.forEach { word ->
if (pattern.matches(word)) {
println("$word matches")
}
}
}
In the example, we add a question mark after the dot character. This means that in the pattern we can have one arbitrary character or we can have no character there.
seven matches even matches
The {n,m} quantifier
The {n,m} quantifier matches at least n and at most m occurrences of the preceding expression.
regex_mn_quantifier.kt
package com.zetcode
fun main() {
val words = listOf("pen", "book", "cool", "pencil", "forest", "car",
"list", "rest", "ask", "point", "eyes")
val pattern = "\\w{3,4}".toRegex()
words.forEach { word ->
if (pattern.matches(word)) {
println("$word matches")
} else {
println("$word does not match")
}
}
}
In the example, we search for words that have either three or four characters.
val pattern = “\w{3,4}".toRegex()
In the pattern, we have a word character repeated three or four times. Note that there must not be a space between the numbers.
pen matches book matches cool matches pencil does not match forest does not match car matches list matches rest matches ask matches point does not match eyes matches
Kotlin regex anchors
Anchors match positions of characters inside a given text. When using the ^ anchor the match must occur at the beginning of the string and when using the $ anchor the match must occur at the end of the string.
regex_anchors.kt
package com.zetcode
fun main() {
val sentences = listOf("I am looking for Jane.",
"Jane was walking along the river.",
"Kate and Jane are close friends.")
val pattern = "^Jane".toRegex()
sentences.forEach { sentence ->
if (pattern.containsMatchIn(sentence)) {
println(sentence)
}
}
}
In the example, we have three sentences. The search pattern is ^Jane. The pattern checks if the “Jane” string is located at the beginning of the text. The Jane. would look for “Jane” at the end of the sentence.
Kotlin regex alternations
The alternation operator | creates a regular expression with several choices.
regex_alternations.kt
package com.zetcode
fun main() {
val words = listOf("Jane", "Thomas", "Robert",
"Lucy", "Beky", "John", "Peter", "Andy")
val pattern = "Jane|Beky|Robert".toRegex()
words.forEach { word ->
if (pattern.matches(word)) {
println(word)
}
}
}
We have eight names in the list.
val pattern = “Jane|Beky|Robert”.toRegex()
This regular expression looks for “Jane”, “Beky”, or “Robert” strings.
Kotlin regex subpatterns
Subpatterns are patterns within patterns. Subpatterns are created with () characters.
regex_subpatterns.kt
package com.zetcode
fun main() {
val words = listOf("book", "bookshelf", "bookworm",
"bookcase", "bookish", "bookkeeper", "booklet", "bookmark")
val pattern = "book(worm|mark|keeper)?".toRegex()
words.forEach { word ->
if (pattern.matches(word)) {
println("$word matches")
} else {
println("$word does not match")
}
}
}
The example creates a subpattern.
val pattern = “book(worm|mark|keeper)?".toRegex()
The regular expression uses a subpattern. It matches bookworm, bookmark, bookkeeper, and book words.
book matches bookshelf does not match bookworm matches bookcase does not match bookish does not match bookkeeper matches booklet does not match bookmark matches
Kotlin regex character classes
A character class defines a set of characters, any one of which can occur in an input string for a match to succeed.
character_classes.kt
package com.zetcode
fun main() {
val words = listOf("a gray bird", "grey hair", "great look")
val pattern = "gr[ea]y".toRegex()
words.forEach { word ->
if (pattern.containsMatchIn(word)) {
println(word)
}
}
}
In the example, we use a character class to include both gray and grey words.
val pattern = “gr[ea]y”.toRegex()
The [ea] class allows to use either ’e’ or ‘a’ character in the pattern.
Kotlin named character classes
There are some predefined character classes. The \s matches a whitespace character [\t\n\t\f\v], the \d a digit [0-9], and the \w a word character [a-zA-Z0-9_].
named_character_classes.kt
package com.zetcode
fun main() {
val text = "We met in 2013. She must be now about 27 years old."
val pattern = "\\d+".toRegex()
val found = pattern.findAll(text)
found.forEach { f ->
val m = f.value
println(m)
}
}
In the example, we search for numbers in the text.
val pattern = “\d+".toRegex()
The \d+ pattern looks for any number of digit sets in the text.
val found = pattern.findAll(text)
To find all the matches with findAll.
2013 27
Kotlin regex capturing groups
Round brackets are used to create capturing groups. This allows us to apply a quantifier to the entire group or to restrict alternation to a part of the regular expression.
capturing_groups.kt
package com.zetcode
fun main() {
val sites = listOf(
"webcode.me", "zetcode.com", "freebsd.org",
"netbsd.org"
)
val pattern = "(\\w+)\\.(\\w+)".toRegex()
for (site in sites) {
val matches = pattern.findAll(site)
matches.forEach { matchResult ->
println(matchResult.value)
println(matchResult.groupValues[1])
println(matchResult.groupValues[2])
println("*****************")
}
}
}
In the example, we divide the domain names into two parts by using groups.
val pattern = “(\w+)\.(\w+)".toRegex()
We define two groups with parentheses.
matches.forEach { matchResult ->
println(matchResult.value)
println(matchResult.groupValues[1])
println(matchResult.groupValues[2])
println("*****************")
}
The groups are accessed via the groupValues function. The groupValues[0]) returns the whole matched string; it is equivalent to the value property.
webcode.me webcode me
zetcode.com zetcode com
freebsd.org freebsd org
netbsd.org netbsd org
In the following example, we use groups to work with expressions.
regex_expressions.kt
package com.zetcode
fun main() {
val expressions = listOf("16 + 11", "12 * 5", "27 / 3", "2 - 8")
val pattern = "(\\d+)\\s+([-+*/])\\s+(\\d+)".toRegex()
for (expression in expressions) {
val matches = pattern.findAll(expression)
matches.forEach { matchResult ->
val value1 = matchResult.groupValues[1].toInt()
val value2 = matchResult.groupValues[3].toInt()
val msg = when (matchResult.groupValues[2]) {
"+" -> "$expression = ${value1 + value2}"
"-" -> "$expression = ${value1 - value2}"
"*" -> "$expression = ${value1 * value2}"
"/" -> "$expression = ${value1 / value2}"
else -> "Unknown operator"
}
println(msg)
}
}
}
The example parses four simple mathematical expressions and computes them.
val expressions = listOf(“16 + 11”, “12 * 5”, “27 / 3”, “2 - 8”)
We have a list of four expressions.
val pattern = “(\d+)\s+([-+*/])\s+(\d+)".toRegex()
In the regex pattern, we have three groups: two groups for the values, one for the operator.
val value1 = matchResult.groupValues[1].toInt() val value2 = matchResult.groupValues[3].toInt()
We get the values and transform them into integers.
val msg = when (matchResult.groupValues[2]) {
"+" -> "$expression = ${value1 + value2}"
"-" -> "$expression = ${value1 - value2}"
"*" -> "$expression = ${value1 * value2}"
"/" -> "$expression = ${value1 / value2}"
else -> "Unknown operator"
}
With the when expression, we compute the expressions and build the messages.
16 + 11 = 27 12 * 5 = 60 27 / 3 = 9 2 - 8 = -6
Kotlin regex word frequency
In the next example, we count the frequency of words in a file.
$ wget https://raw.githubusercontent.com/janbodnar/data/main/the-king-james-bible.txt
We use the King James Bible.
word_freq.kt
import java.io.File
fun main() {
val fileName = "src/main/resources/the-king-james-bible.txt";
val text = File(fileName).readText()
val r = "[a-zA-Z']+".toRegex()
val matches = r.findAll(text)
val data = matches.map { it.value }
.groupBy { it }
.map { Pair(it.key, it.value.size) }
.sortedByDescending { it.second }
.take(10)
for ((word, freq) in data) {
System.out.printf("%s %d \n", word, freq)
}
}
We find all matching words with findAll. We group the words and sort them by the number of times they are present. We print the first top words.
the 62103 and 38848 of 34478 to 13400 And 12846 that 12576 in 12331 shall 9760 he 9665 unto 8942
Kotlin regex email example
In the following example, we create a regex pattern for checking email addresses.
regex_emails.kt
package com.zetcode
fun main() {
val emails = listOf("luke@gmail.com", "andy@yahoocom",
"34234sdfa#2345", "f344@gmail.com", "dandy!@yahoo.com")
val pattern = "[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\\.[a-zA-Z.]{2,18}".toRegex()
emails.forEach { email ->
if (pattern.matches(email)) {
println("$email matches")
} else {
println("$email does not match")
}
}
}
This example provides one possible solution.
val pattern = “[a-zA-Z0-9._-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,18}".toRegex()
The email is divided into five parts. The first part is the local part. Usually it is a name of a company, an individual, or a nickname. The [a-zA-Z0-9._-]+ lists all possible characters that we can use in the local part. They can be used one or more times.
The second part consists of the literal @ character. The third part is the domain part. It is usually the domain name of the email provider such as yahoo, or gmail. The [a-zA-Z0-9-]+ is a character class providing all characters that can be used in the domain name. The + quantifier allows to use of one or more of these characters.
The fourth part is the dot character; it is preceded by double escape character (\) to get a literal dot.
The final part is the top level domain name: [a-zA-Z.]{2,18}. Top level domains can have from 2 to 18 characters, such as sk, net, info, travel, cleaning, travelinsurance. The maximum length can be 63 characters, but most domain are shorter than 18 characters today. There is also a dot character. This is because some top level domains have two parts; for instance co.uk.
luke@gmail.com matches andy@yahoocom does not match 34234sdfa#2345 does not match f344@gmail.com matches dandy!@yahoo.com does not match
Source
Kotlin regular expressions documentation
In this chapter, we have covered regular expressions in Kotlin.
Author
My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.
List all Kotlin tutorials.
