Find byte offset of a pattern in Golang - go

Question

We can find the byte offset of a pattern from file by
"grep -ob pattern filename";
However, grep is not utf8 safe.
How do I find byte offset of a pattern in Go? The file is process log, which can be in TB.
This is what I want to get in Go:
$ cat fname
hello world
findme
hello 世界
findme again
...
$ grep -ob findme fname
12:findme
32:findme

Answer 1

FindAllStringIndex(s string, n int) returns byte start/finish indexes (i.e., slices) of all successive matches of the expression:
package main
import "fmt"
import "io/ioutil"
import "regexp"
func main() {
fname := "C:\\Users\\UserName\\go\\src\\so56798431\\fname"
b, err := ioutil.ReadFile(fname)
if err != nil {
panic(err)
}
re, err := regexp.Compile("findme")
if err != nil {
// handle error
}
fmt.Println(re.FindAllStringIndex(string(b), -1))
}
Output:
[[12 18] [32 38]]
Note: I did this on Microsoft Windows, but saved the file in UNIX format (linefeed); if input file saved in Windows format (carriage return & linefeed) the byte offsets would increment to 13 and 35, respectively.
UPDATE: for large files, use bufio.Scanner; for example:
package main
import (
"bufio"
"fmt"
"log"
"os"
"regexp"
)
func main() {
fname, err := os.Open("C:\\Users\\UserName\\go\\src\\so56798431\\fname")
if err != nil {
log.Fatal(err)
}
defer fname.Close()
re, err := regexp.Compile("findme")
if err != nil {
// handle error
}
scanner := bufio.NewScanner(fname)
bytesRead := 0
for scanner.Scan() {
b := scanner.Text()
//fmt.Println(b)
results := re.FindAllStringIndex(b, -1)
for _, result := range results {
fmt.Println(bytesRead + result[0])
}
// account for UNIX EOL marker
bytesRead += len(b) + 1
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
}
Output:
12
32