Regex Recompiled In Loop (ARCH5073)

Key go.performance.regex-recompiled-in-loop · Severity warning · Category performance

Calling regexp.MustCompile inside a loop recompiles the same pattern on every iteration, which is expensive. Compile the pattern once outside the loop.

Incorrect

func filter(lines []string) []string {
  var out []string
  for _, line := range lines {
    re := regexp.MustCompile(`^\d+`)
    if re.MatchString(line) {
      out = append(out, line)
    }
  }
  return out
}

Correct

var numRe = regexp.MustCompile(`^\d+`)

func filter(lines []string) []string {
  var out []string
  for _, line := range lines {
    if numRe.MatchString(line) {
      out = append(out, line)
    }
  }
  return out
}

Fix

Compile the regex once outside the loop, ideally as a package-level var.