How to get capturing group functionality in Go regular expressions
RegexGoCapture GroupRegex Problem Overview
I'm porting a library from Ruby to Go, and have just discovered that regular expressions in Ruby are not compatible with Go (google RE2). It's come to my attention that Ruby & Java (plus other languages use PCRE regular expressions (perl compatible, which supports capturing groups)), so I need to re-write my expressions so that they compile ok in Go.
For example, I have the following regex:
`(?<Year>\d{4})-(?<Month>\d{2})-(?<Day>\d{2})`
This should accept input such as:
2001-01-20
The capturing groups allow the year, month and day to be captured into variables. To get the value of each group, it's very easy; you just index into the returned matched data with the group name and you get the value back. So, for example to get the year, something like this pseudo code:
m=expression.Match("2001-01-20")
year = m["Year"]
This is a pattern I use a lot in my expressions, so I have a lot of re-writing to do.
So, is there a way to get this kind of functionality in Go regexp; how should I re-write these expressions?
Regex Solutions
Solution 1 - Regex
> how should I re-write these expressions?
Add some Ps, as defined here:
(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})
Cross reference capture group names with re.SubexpNames().
And use as follows:
package main
import (
"fmt"
"regexp"
)
func main() {
r := regexp.MustCompile(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`)
fmt.Printf("%#v\n", r.FindStringSubmatch(`2015-05-27`))
fmt.Printf("%#v\n", r.SubexpNames())
}
Solution 2 - Regex
I had created a function for handling url expressions but it suits your needs too. You can check this snippet but it simply works like this:
/**
* Parses url with the given regular expression and returns the
* group values defined in the expression.
*
*/
func getParams(regEx, url string) (paramsMap map[string]string) {
var compRegEx = regexp.MustCompile(regEx)
match := compRegEx.FindStringSubmatch(url)
paramsMap = make(map[string]string)
for i, name := range compRegEx.SubexpNames() {
if i > 0 && i <= len(match) {
paramsMap[name] = match[i]
}
}
return paramsMap
}
You can use this function like:
params := getParams(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`, `2015-05-27`)
fmt.Println(params)
and the output will be:
map[Year:2015 Month:05 Day:27]
Solution 3 - Regex
To improve RAM and CPU usage without calling anonymous functions inside loop and without copying arrays in memory inside loop with "append" function see the next example:
You can store more than one subgroup with multiline text, without appending string with '+' and without using for loop inside for loop (like other examples posted here).
txt := `2001-01-20
2009-03-22
2018-02-25
2018-06-07`
regex := *regexp.MustCompile(`(?s)(\d{4})-(\d{2})-(\d{2})`)
res := regex.FindAllStringSubmatch(txt, -1)
for i := range res {
//like Java: match.group(1), match.gropu(2), etc
fmt.Printf("year: %s, month: %s, day: %s\n", res[i][1], res[i][2], res[i][3])
}
Output:
year: 2001, month: 01, day: 20
year: 2009, month: 03, day: 22
year: 2018, month: 02, day: 25
year: 2018, month: 06, day: 07
Note: res[i][0] =~ match.group(0) Java
If you want to store this information use a struct type:
type date struct {
y,m,d int
}
...
func main() {
...
dates := make([]date, 0, len(res))
for ... {
dates[index] = date{y: res[index][1], m: res[index][2], d: res[index][3]}
}
}
It's better to use anonymous groups (performance improvement)
Using "ReplaceAllGroupFunc" posted on Github is bad idea because:
- is using loop inside loop
- is using anonymous function call inside loop
- has a lot of code
- is using the "append" function inside loop and that's bad. Every time a call is made to "append" function, is copying the array to new memory position
Solution 4 - Regex
As of GO 1.15, you can simplify the process by using Regexp.SubexpIndex. You can check the release notes at https://golang.org/doc/go1.15#regexp.
Based in your example, you'd have something like the following:
re := regexp.MustCompile(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`)
matches := re.FindStringSubmatch("Some random date: 2001-01-20")
yearIndex := re.SubexpIndex("Year")
fmt.Println(matches[yearIndex])
You can check and execute this example at https://play.golang.org/p/ImJ7i_ZQ3Hu.
Solution 5 - Regex
Simple way to determine group names based on @VasileM answer.
Disclaimer: it's not about memory/cpu/time optimization
package main
import (
"fmt"
"regexp"
)
func main() {
r := regexp.MustCompile(`^(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})$`)
res := r.FindStringSubmatch(`2015-05-27`)
names := r.SubexpNames()
for i, _ := range res {
if i != 0 {
fmt.Println(names[i], res[i])
}
}
}
Solution 6 - Regex
If you need to replace based on a function while capturing groups you can use this:
import "regexp"
func ReplaceAllGroupFunc(re *regexp.Regexp, str string, repl func([]string) string) string {
result := ""
lastIndex := 0
for _, v := range re.FindAllSubmatchIndex([]byte(str), -1) {
groups := []string{}
for i := 0; i < len(v); i += 2 {
groups = append(groups, str[v[i]:v[i+1]])
}
result += str[lastIndex:v[0]] + repl(groups)
lastIndex = v[1]
}
return result + str[lastIndex:]
}
Example:
str := "abc foo:bar def baz:qux ghi"
re := regexp.MustCompile("([a-z]+):([a-z]+)")
result := ReplaceAllGroupFunc(re, str, func(groups []string) string {
return groups[1] + "." + groups[2]
})
fmt.Printf("'%s'\n", result)
Solution 7 - Regex
You can use regroup library for that
https://github.com/oriser/regroup
Example:
package main
import (
"fmt"
"github.com/oriser/regroup"
)
func main() {
r := regroup.MustCompile(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`)
mathces, err := r.Groups("2015-05-27")
if err != nil {
panic(err)
}
fmt.Printf("%+v\n", mathces)
}
Will print: map[Year:2015 Month:05 Day:27]
Alternatively, you can use it like this:
package main
import (
"fmt"
"github.com/oriser/regroup"
)
type Date struct {
Year int `regroup:"Year"`
Month int `regroup:"Month"`
Day int `regroup:"Day"`
}
func main() {
date := &Date{}
r := regroup.MustCompile(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`)
if err := r.MatchToTarget("2015-05-27", date); err != nil {
panic(err)
}
fmt.Printf("%+v\n", date)
}
Will print: &{Year:2015 Month:5 Day:27}
Solution 8 - Regex
Function for get regexp parameters wit nil pointer checking. Returns map[] if error ocured
// GetRxParams - Get all regexp params from string with provided regular expression
func GetRxParams(rx *regexp.Regexp, str string) (pm map[string]string) {
if !rx.MatchString(str) {
return nil
}
p := rx.FindStringSubmatch(str)
n := rx.SubexpNames()
pm = map[string]string{}
for i := range n {
if i == 0 {
continue
}
if n[i] != "" && p[i] != "" {
pm[n[i]] = p[i]
}
}
return
}