I'm trying to find the begin of a named capturing groups in a string to create a simple parser (see related question). To do this the extract
function remembers the last for characters in the last4
variable. If the last 4 characters are equal to "(?P<" it is the beginning of a capturing group:
package main
import "fmt"
const sample string = `/(?P<country>m((a|b).+)(x|y)n)/(?P<city>.+)`
func main() {
extract(sample)
}
func extract(regex string) {
last4 := new([4]int32)
for _, c := range regex {
last4[0], last4[1], last4[2], last4[3] = last4[1], last4[2], last4[3], c
last4String := fmt.Sprintf("%c%c%c%c\n", last4[0], last4[1], last4[2], last4[3])
if last4String == "(?P<" {
fmt.Print("start of capturing group")
}
}
}
http://play.golang.org/p/pqA-wCuvux
But this code prints nothing! last4String == "(?P<"
is never true, although this substrin appears in the output if I print last4String
inside the loop. How to compare strings in Go then?
And is there a more elegant way to convert an int32 array to a string than fmt.Sprintf("%c%c%c%c\n", last4[0], last4[1], last4[2], last4[3])
?
Anything else that could be better? My code looks somewhat inelegant to me.
If it's not for self-education or similar, you probably want to use the existing RE parser in the standard library and then "walk" the AST to do whatever required.
func Parse(s string, flags Flags) (*Regexp, error)
Parse parses a regular expression string s, controlled by the specified Flags, and returns a regular expression parse tree. The syntax is described in the top-level comment for package regexp.
There's even a helper for your task.
EDIT1: Your code repaired:
package main
import "fmt"
const sample string = `/(?P<country>m((a|b).+)(x|y)n)/(?P<city>.+)`
func main() {
extract(sample)
}
func extract(regex string) {
var last4 [4]int32
for _, c := range regex {
last4[0], last4[1], last4[2], last4[3] = last4[1], last4[2], last4[3], c
last4String := fmt.Sprintf("%c%c%c%c", last4[0], last4[1], last4[2], last4[3])
if last4String == "(?P<" {
fmt.Println("start of capturing group")
}
}
}
(Also here)
EDIT2: Your code rewritten:
package main
import (
"fmt"
"strings"
)
const sample string = `/(?P<country>m((a|b).+)(x|y)n)/(?P<city>.+)`
func main() {
extract(sample)
}
func extract(regex string) {
start := 0
for {
i := strings.Index(regex[start:], "(?P<")
if i < 0 {
break
}
fmt.Printf("start of capturing group @ %d\n", start+i)
start += i + 1
}
}
(Also here)
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