How to select first chars with a custom word boundary? - regex

Question

I've test cases with a series of words like this :
{
input: "Halley's Comet",
expected: "HC",
},
{
input: "First In, First Out",
expected: "FIFO",
},
{
input: "The Road _Not_ Taken",
expected: "TRNT",
},
I want with one regex to match all first letters of these words, avoid char: "_" to be matched as a first letter and count single quote in the word.
Currently, I have this regex working on pcre syntax but not with Go regexp package : (?<![a-zA-Z0-9'])([a-zA-Z0-9'])
I know lookarounds aren't supported by Go but I'm looking for a good way to do that.
I also use this func to get an array of all strings : re.FindAllString(s, -1)
Thanks for helping.

Answer 1

Something that plays with character classes and word boundaries should suffice:
\b_*([a-z])[a-z]*(?:'s)?_*\b\W*
demo
Usage:
package main
import (
"fmt"
"regexp"
)
func main() {
re := regexp.MustCompile(`(?i)\b_*([a-z])[a-z]*(?:'s)?_*\b\W*`)
fmt.Println(re.ReplaceAllString("O'Brian's dog", "$1"))
}

Answer 2

ftr, regexp less solution
package main
import (
"fmt"
)
func main() {
inputs := []string{"Hallمرحباey's Comet", "First In, First Out", "The Road _Not_ Taken", "O'Brian's Dog"}
c := [][]string{}
w := [][]string{}
for _, input := range inputs {
c = append(c, firstLet(input))
w = append(w, words(input))
}
fmt.Printf("%#v\n", w)
fmt.Printf("%#v\n", c)
}
func firstLet(in string) (out []string) {
var inword bool
for _, r := range in {
if !inword {
if isChar(r) {
inword = true
out = append(out, string(r))
}
} else if r == ' ' {
inword = false
}
}
return out
}
func words(in string) (out []string) {
var inword bool
var w []rune
for _, r := range in {
if !inword {
if isChar(r) {
w = append(w, r)
inword = true
}
} else if r == ' ' {
if len(w) > 0 {
out = append(out, string(w))
w = w[:0]
}
inword = false
} else if r != '_' {
w = append(w, r)
}
}
if len(w) > 0 {
out = append(out, string(w))
}
return out
}
func isChar(r rune) bool {
return (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z')
}
outputs
[][]string{[]string{"Hallمرحباey's", "Comet"}, []string{"First", "In,", "First", "Out"}, []string{"The", "Road", "Not", "Taken"}, []string{"O'Brian's", "Dog"}}
[][]string{[]string{"H", "C"}, []string{"F", "I", "F", "O"}, []string{"T", "R", "N", "T"}, []string{"O", "D"}}