Go regular expressions
last modified April 11, 2024
In this article we show how to parse text in Go using regular expressions.
$ go version go version go1.22.2 linux/amd64
We use Go version 1.22.2.
Regular expressions
Regular expressions are used for text searching and more advanced text manipulation. Regular expressions are built into tools including grep and sed, text editors including vi and emacs, programming languages including Go, Java, and Python.
Go has built-in API for working with regular expressions; it is located in
regexp package.
A regular expression defines a search pattern for strings. It is used to match text, replace text, or split text. A regular expression may be compiled for better performance. The Go syntax of the regular expressions accepted is the same general syntax used by Perl, Python, and other languages.
Regex examples
The following table shows a couple of regular expression strings.
| 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] |
Go regex MatchString
The MatchString function reports whether a string contains any
match of the regular expression pattern.
package main
import (
"fmt"
"log"
"regexp"
)
func main() {
words := [...]string{"Seven", "even", "Maven", "Amen", "eleven"}
for _, word := range words {
found, err := regexp.MatchString(".even", word)
if err != nil {
log.Fatal(err)
}
if found {
fmt.Printf("%s matches\n", word)
} else {
fmt.Printf("%s does not match\n", word)
}
}
}
In the code example, we have five words in an array. We check which words match
the .even regular expression.
words := [...]string{"Seven", "even", "Maven", "Amen", "eleven"}
We have an array of words.
for _, word := range words {
We go through the array of words.
found, err := regexp.MatchString(".even", word)
We check if the current word matches the regular expression with MatchString.
We have the .even regular expression. The dot (.) metacharacter
stands for any single character in the text.
if found {
fmt.Printf("%s matches\n", word)
} else {
fmt.Printf("%s does not match\n", word)
}
We print if of the word matches the regular expression or not.
$ go run matchstring.go Seven matches even does not match Maven does not match Amen does not match eleven matches
Two words in the array match the our regular expression.
Go compiled regular expression
The Compile function parses a regular expression and returns, if
successful, a Regexp object that can be used to match against text.
Compiled regular expressions yield faster code.
The MustCompile function is a convenience function which
compiles a regular expression and panics if the expression cannot be parsed.
package main
import (
"fmt"
"log"
"regexp"
)
func main() {
words := [...]string{"Seven", "even", "Maven", "Amen", "eleven"}
re, err := regexp.Compile(".even")
if err != nil {
log.Fatal(err)
}
for _, word := range words {
found := re.MatchString(word)
if found {
fmt.Printf("%s matches\n", word)
} else {
fmt.Printf("%s does not match\n", word)
}
}
}
In the code example, we used a compiled regular expression.
re, err := regexp.Compile(".even")
We compile the regular expression with Compile.
found := re.MatchString(word)
The MatchString function is called on the returned regex object.
package main
import (
"fmt"
"regexp"
)
func main() {
words := [...]string{"Seven", "even", "Maven", "Amen", "eleven"}
re := regexp.MustCompile(".even")
for _, word := range words {
found := re.MatchString(word)
if found {
fmt.Printf("%s matches\n", word)
} else {
fmt.Printf("%s does not match\n", word)
}
}
}
The example is simplified with MustCompile.
Go regex FindAllString
The FindAllString function returns a slice of all successive
matches of the regular expression.
package main
import (
"fmt"
"os"
"regexp"
)
func main() {
var 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.`
re := regexp.MustCompile("(?i)fox(es)?")
found := re.FindAllString(content, -1)
fmt.Printf("%q\n", found)
if found == nil {
fmt.Printf("no match found\n")
os.Exit(1)
}
for _, word := range found {
fmt.Printf("%s\n", word)
}
}
In the code example, we find all occurrences of the word fox, including its plural form.
re := regexp.MustCompile("(?i)fox(es)?")
With the (?i) syntax, the regular expression is case insensitive.
The (es)? indicates that "es" characters might be included zero
times or once.
found := re.FindAllString(content, -1)
We look for all occurrences of the defined regular expression with FindAllString.
The second parameter is the maximum matches to look for; -1 means search for all
possible matches.
$ go run findall.go ["Foxes" "Foxes" "Foxes" "fox" "fox"] Foxes Foxes Foxes fox fox
We have found five matches.
Go regex FindAllStringIndex
The FindAllStringIndex returns a slice of all successive indexes of
matches of the expression.
package main
import (
"fmt"
"regexp"
)
func main() {
var 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.`
re := regexp.MustCompile("(?i)fox(es)?")
idx := re.FindAllStringIndex(content, -1)
for _, j := range idx {
match := content[j[0]:j[1]]
fmt.Printf("%s at %d:%d\n", match, j[0], j[1])
}
}
In the code example, we find all occurrences of the fox word and their indexes in the text.
$ go run allindex.go Foxes at 0:5 Foxes at 81:86 Foxes at 196:201 fox at 296:299 fox at 311:314
Go regex Split
The Split function cuts a string into substrings separated by the
defined regular expression. It returns a slice of the substrings between those
expression matches.
package main
import (
"fmt"
"log"
"regexp"
"strconv"
)
func main() {
var data = `22, 1, 3, 4, 5, 17, 4, 3, 21, 4, 5, 1, 48, 9, 42`
sum := 0
re := regexp.MustCompile(",\\s*")
vals := re.Split(data, -1)
for _, val := range vals {
n, err := strconv.Atoi(val)
sum += n
if err != nil {
log.Fatal(err)
}
}
fmt.Println(sum)
}
In the code example, we have a comma-separated list of values. We cut the values from the string and calculate their sum.
re := regexp.MustCompile(",\\s*")
The regular expresion includes a comma character and any number of adjacend spaces.
vals := re.Split(data, -1)
We get the slice of values.
for _, val := range vals {
n, err := strconv.Atoi(val)
sum += n
if err != nil {
log.Fatal(err)
}
}
We go through the slice and calculate the sum. The slice contains strings; therefore,
we convert each string into an integer with strconv.Atoi function.
$ go run splittext.go 189
The sum of values is 189.
Go 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.
To find capturing groups (Go uses the term subexpressions), we use the
FindStringSubmatch function.
package main
import (
"fmt"
"regexp"
)
func main() {
websites := [...]string{"webcode.me", "zetcode.com", "freebsd.org", "netbsd.org"}
re := regexp.MustCompile("(\\w+)\\.(\\w+)")
for _, website := range websites {
parts := re.FindStringSubmatch(website)
for i, _ := range parts {
fmt.Println(parts[i])
}
fmt.Println("---------------------")
}
}
In the code example, we divide the domain names into two parts by using groups.
re := regexp.MustCompile("(\\w+)\\.(\\w+)")
We define two groups with parentheses.
parts := re.FindStringSubmatch(website)
The FindStringSubmatch returns a slice of strings holding the
matches, including those from the capturing groups.
$ go run capturegroups.go webcode.me webcode me --------------------- zetcode.com zetcode com --------------------- freebsd.org freebsd org --------------------- netbsd.org netbsd org ---------------------
Go regex replacing strings
It is possible to replace strings with ReplaceAllString. The method
returns the modified string.
package main
import (
"fmt"
"io/ioutil"
"log"
"net/http"
"regexp"
"strings"
)
func main() {
resp, err := http.Get("http://webcode.me")
if err != nil {
log.Fatal(err)
}
defer resp.Body.Close()
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
log.Fatal(err)
}
content := string(body)
re := regexp.MustCompile("<[^>]*>")
replaced := re.ReplaceAllString(content, "")
fmt.Println(strings.TrimSpace(replaced))
}
The example reads HTML data of a web page and strips its HTML tags using a regular expression.
resp, err := http.Get("http://webcode.me")
We create a GET request with Get function from the http
package.
body, err := ioutil.ReadAll(resp.Body)
We read the body of the response object.
re := regexp.MustCompile("<[^>]*>")
This pattern defines a regular expression that matches HTML tags.
replaced := re.ReplaceAllString(content, "")
We remove all the tags with ReplaceAllString method.
Go regex ReplaceAllStringFunc
The ReplaceAllStringFunc returns a copy of a string in which all
matches of the regular expression have been replaced by the return value of
the specified function.
package main
import (
"fmt"
"regexp"
"strings"
)
func main() {
content := "an old eagle"
re := regexp.MustCompile(`[^aeiou]`)
fmt.Println(re.ReplaceAllStringFunc(content, strings.ToUpper))
}
In the code example, we apply the strings.ToUpper function on all
wovels of a string.
$ go run replaceallfunc.go aN oLD eaGLe
Source
In this article we have worked with regular expression in Go.
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