How Can I Use Capturing Groups in Go Regular Expressions?

Capturing Groups in Go Regular Expressions

When porting code from Ruby to Go, developers often encounter compatibility issues with regular expressions. Ruby supports capturing groups within regexes, a feature not natively available in Go's RE2 library.

Rewriting Expressions for Go

To emulate capturing group functionality, the following steps can be taken:

1. Use Named Capture Groups:
Instead of using angle brackets, add "P" between the opening parenthesis and the group name, e.g., (?Pd{4}) for the year group.

2. Cross-Reference Group Names:
Utilize the regexp.SubexpNames() function to obtain a list of capture group names.

3. Access Captured Data:
Use the appropriate group name as an argument to functions like FindStringSubmatch to retrieve the captured values.

Example:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    r := regexp.MustCompile(`(?P<Year>\d{4})-(?P<Month>\d{2})-(?P<Day>\d{2})`)
    s := `2015-05-27`
    fmt.Printf("Matched Substrings: %#v\n", r.FindStringSubmatch(s))
    fmt.Printf("Group Names: %#v\n", r.SubexpNames())
}

Output:

Matched Substrings: [2015 05 27]
Group Names: [ Year Month Day ]

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