Go Regexp: Understanding Any Character Match

The Go re2 syntax document states that the any character (.) matches any character, including newline when the "s" flag is set. However, a recent query raised concerns as a test program seemed to indicate otherwise.

Program Results Unexpected

The provided program (http://play.golang.org/p/pccP52RvKS) aims to match all characters, including newline, but its results suggest the any character is not matching newline.

Addressing the Discrepancy

Like many other regex engines, Go's re2 does not match newlines with the "." metacharacter by default. To enable newline matching, the "?s" (dot all) flag must be added to the regex.

Example with "?s" Flag

A modified version of the test program incorporating the "?s" flag:

package main

import (
    "fmt"
    "regexp"
)

func main() {
    re := regexp.MustCompile("(?s). ")
    match := re.FindString("abc\ndef")
    fmt.Println(match)
}

When executed, this program correctly prints "abc\ndef," demonstrating that the any character now matches newline as expected.

Conclusion

In Go's re2 syntax, the "." metacharacter does not inherently match newline. To enable newline matching, the "?s" flag must be added to the regex. By incorporating this flag, users can ensure accurate matching behavior that aligns with the re2 syntax documentation.