如何在Go中使用/访问捕获组？ [重复]

Question

This question already has an answer here:

• Regex with replace in Golang 1 answer

I have a file with dates in the format dd.mm.yyyy (e.g. 31.12.2019). I want to transform into format yyyy-mm-dd (e.g. 2019-12-31).

In Notepad++ I can do a Search and Replace with these strings using back references:

Search:

(\d{2}).(\d{2}).(\d{4})

Replace:

\3-\2-\1

How would I do this with Go?

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Answer 1

You could do it by slicing your input string, and assembling the parts in different order:

func transform(s string) string {
    d, m, y := s[:2], s[3:5], s[6:]
    return y + "-" + m + "-" + d
}

Note: the above function does not validate the input, it could panic if the input is shorter than 6 bytes.

If you need input validation (including date validation), you could use the time package to parse the date, and format it into your expected output:

func transform2(s string) (string, error) {
    t, err := time.Parse("02.01.2006", s)
    if err != nil {
        return "", err
    }
    return t.Format("2006-01-02"), nil
}

Testing the above functions:

fmt.Println(transform("31.12.2019"))
fmt.Println(transform2("31.12.2019"))

Output (try it on the Go Playground):

2019-12-31
2019-12-31 <nil>