在Golang中提取部分字符串？

I'm learning Golang so I can rewrite some of my shell scripts.

I have URL's that look like this:

https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value

I want to extract the following part:

https://example-1.example.com/a/c482dfad3573acff324c/list.txt

In a shell script I would do something like this:

echo "$myString" | grep -o 'http://.*.txt'

What is the best way to do the same thing in Golang, only by using the standard library?

There are a few options:

// match regexp as in question
pat := regexp.MustCompile(`https?://.*\.txt`)
s := pat.FindString(myString)

// everything before the query 
s := strings.Split(myString, "?")[0] string

// same as previous, but avoids []string allocation
s := myString
if i := strings.IndexByte(s, '?'); i >= 0 {
    s = s[:i]
}

// parse and clear query string
u, err := url.Parse(myString)
u.RawQuery = ""
s := u.String()

The last option is the best because it will handle all possible corner cases.

try it on the playground

you may use strings.IndexRune, strings.IndexByte, strings.Split, strings.SplitAfter, strings.FieldsFunc, url.Parse, regexp or your function.

first most simple way:
you may use i := strings.IndexRune(s, '?') or i := strings.IndexByte(s, '?') then s[:i] like this (with commented output):

package main

import "fmt"
import "strings"

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    i := strings.IndexByte(s, '?')
    if i != -1 {
        fmt.Println(s[:i]) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
    }
}

or you may use url.Parse(s) (I'd use this):

package main

import "fmt"
import "net/url"

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    url, err := url.Parse(s)
    if err == nil {
        url.RawQuery = ""
        fmt.Println(url.String()) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
    }
}

or you may use regexp.MustCompile(".*\\.txt"):

package main

import "fmt"
import "regexp"

var rgx = regexp.MustCompile(`.*\.txt`)

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`

    fmt.Println(rgx.FindString(s)) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
}

or you may use splits := strings.FieldsFunc(s, func(r rune) bool { return r == '?' }) then splits[0]:

package main

import "fmt"
import "strings"

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    splits := strings.FieldsFunc(s, func(r rune) bool { return r == '?' })
    fmt.Println(splits[0]) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
}

you may use splits := strings.Split(s, "?") then splits[0]:

package main

import "fmt"
import "strings"

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    splits := strings.Split(s, "?")
    fmt.Println(splits[0]) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
}

you may use splits := strings.SplitAfter(s, ".txt") then splits[0]:

package main

import "fmt"
import "strings"

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    splits := strings.SplitAfter(s, ".txt")
    fmt.Println(splits[0]) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
}

or you may use your function (most independent way):

package main

import "fmt"

func left(s string) string {
    for i, r := range s {
        if r == '?' {
            return s[:i]
        }
    }
    return ""
}

func main() {
    s := `https://example-1.example.com/a/c482dfad3573acff324c/list.txt?parm1=value,parm2=value,parm3=https://example.com/a?parm1=value,parm2=value`
    fmt.Println(left(s)) // https://example-1.example.com/a/c482dfad3573acff324c/list.txt
}

If you are prosessing only URLs, you can use Go's net/url library https://golang.org/pkg/net/url/ to parse the URL, truncate the Query and Fragment parts (Query would be parm1=value,parm2=value etc.), and extract the remaining portion scheme://host/path, as in the following example (https://play.golang.org/p/Ao0jU22NyA):

package main

import (
    "fmt"
    "net/url"
)

func main() {
    u, _ := url.Parse("https://example-1.example.com/a/b/c/list.txt?parm1=value,parm2=https%3A%2F%2Fexample.com%2Fa%3Fparm1%3Dvalue%2Cparm2%3Dvalue#somefragment")
    u.RawQuery, u.Fragment = "", ""
    fmt.Printf("%s
", u)
}

Output:

https://example-1.example.com/a/b/c/list.txt