Sscanf not properly matching single quotes - string

Question

I working on some Go code but I am having troubles figuring out why my string isn't being scanned correctly.
I'm given a string that looks like this:
"ERROR: 1: something happened 'here'"
I'm trying to scan it like this:
n, err := fmt.Sscanf("ERROR: 1: something happened 'here'", "ERROR: 1: something happened '%50s'", &value)
However, every time I check the result of the value, I get something like this:
here'
Where the last single quote is left in.
Any idea how to fix this? I figured this case wouldn't be non-deterministic because the function can't complete formatting without including the quote.
Of course, I can simply remove the last character, but I would prefer a fmt-based solution.

Answer 1

The builtin fmt.Scanner has no way to do what you're trying to do. If the target text was wrapped in double quotes, you could use the %q specifier.
Alternatively, if the target was a single wrapped character, you could use text/scanner. But because your target is neither of those, there's nothing built in. So, your options are regexp, or bufio with a custom scanner, or even just strings.Split. If you insist on using fmt you can do a custom scanner, but it's probably the worst option of everything:
package main
import "fmt"
type quote struct { tok string }
func (q *quote) Scan(state fmt.ScanState, verb rune) error {
tok, err := state.Token(false, func(r rune) bool {
return r != 0x27 // '
})
if err != nil {
return err
}
if _, _, err := state.ReadRune(); err != nil {
if len(tok) == 0 {
panic(err)
}
}
q.tok = string(tok)
return nil
}
Example:
package main
import (
"fmt"
"strings"
)
func main() {
r := strings.NewReader("ERROR: 1: something happened 'here'")
for {
var q quote
_, err := fmt.Fscan(r, &q)
if err != nil {
break
}
fmt.Printf("%q\n", q.tok)
}
}
Result:
"ERROR: 1: something happened "
"here"