Sentence Boundary in GO
Go (RE2) can't run this pattern out of the box.
Try it in the GO tester →Why it doesn't work in GO
Go's RE2 engine doesn't support lookarounds (`(?=...)`, `(?<=...)`, etc.) — they break the linear-time matching guarantee.
Approach
Restructure to capture the surrounding context as a group instead, or use JS / Python where lookarounds are available.
Workaround code in Go (RE2)
goGo (workaround)
package main
import (
"fmt"
"regexp"
"unicode"
)
// RE2 doesn't support lookarounds. The fix: match a BROADER candidate
// without the lookaround, then verify the condition in Go code.
//
// Example: instead of `(?=.*\d)[A-Za-z\d]{8,}` (require a digit),
// match the candidate then check `HasDigit(s)` separately.
func HasDigit(s string) bool {
for _, r := range s {
if unicode.IsDigit(r) {
return true
}
}
return false
}
func main() {
re := regexp.MustCompile(`[A-Za-z\d]{8,}`) // simplified, no lookaround
input := "Password1 weakpass StrongerOne9"
for _, candidate := range re.FindAllString(input, -1) {
if HasDigit(candidate) { // the lookaround condition, in code
fmt.Println(candidate)
}
}
}Match a broader candidate without the lookaround, then verify the constraint in Go. Keeps the linear-time guarantee.
Pattern
regexGO
[.!?]\s+(?=[A-Z]) (flags: g)How the pattern works
`[.!?]` matches terminating punctuation. `\s+` matches whitespace. `(?=[A-Z])` is a lookahead for a capital letter (marks where the next sentence begins).
Examples
Input
Hello world. How are you? I'm fine!Matches
.?