Regex Recompiled In Loop (ARCH3073)

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

Reconstructing the same regex pattern on every iteration adds needless compilation cost; a constant pattern should be compiled once outside the loop.

Incorrect

function findMatches(lines: string[]): string[] {
  const result: string[] = [];
  for (const line of lines) {
    const re = new RegExp("\\d{3}-\\d{4}");
    if (re.test(line)) result.push(line);
  }
  return result;
}

Correct

const PHONE_RE = /\d{3}-\d{4}/;

function findMatches(lines: string[]): string[] {
  const result: string[] = [];
  for (const line of lines) {
    if (PHONE_RE.test(line)) result.push(line);
  }
  return result;
}

Fix

Compile constant regex patterns once outside the loop and reuse them.