正则表达式-如何在出现X个模式后匹配子字符串？

Question

ab1-cde23-fg45-h6-ijk-789.lmn.local.        86400 IN A  12.34.5.123

In the follow DNS entry, I'm trying to match the h6 section (position 4). At this point, I know this section of the domain is only composed of 2 letters/digits or one of each, so I can match it (in a clumsy way) with

"-[a-zA-Z0-9]{2}-"

In a case where I could not assume that this is the only section of a domain with 2 letters/digits, how could I match only the content of the 4th position minus the -? (ab1 being the first position, cde23 the second, and so on, with all the positions separated by -)

I'm able to match up to the 4th positions with the following regex, but it includes everything from the start.

"([a-zA-Z0-9]*-){3}[a-zA-Z0-9]*-"

I'm using theses regexp in golang.

Answer 1

Do:

^(?:[^-]+-){3}([^-]+)

• ^(?:[^-]+-){3} matches - separated first 3 fields, (?:) makes the group non-capturing

• The captured group, ([^-]+) will contain the - separated 4th field.

Demo

---

While we are at this, you should perhaps look at string manipulation rather than costly Regex implementation, plain strings.Split() should do:

package main

import (
    "fmt"
    "strings"
)

func main() {
    s := "ab1-cde23-fg45-h6-ijk-789.lmn.local.        86400 IN A  12.34.5.123"
    fmt.Println(strings.Split(s, "-")[3])
}