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Regex Matcher

Extract all matched text from a large block of text with a regular expression.

Regex Matcher​

Regex Matcher

How it works​

The regex is compiled with the global (g) flag, so every occurrence in the text is collected:

const re = new RegExp(pattern, "g");
const list = [];
let m;
while ((m = re.exec(text)) !== null) {
  list.push(m[0]);
  if (m[0] === "") re.lastIndex++;
}

RegExp.prototype.exec() keeps an internal lastIndex position and returns the next match each time it is called, so looping over it walks through the whole string.

The if (m[0] === "") re.lastIndex++ line guards against zero-length matches (for example the pattern a*): without it, exec() would keep returning an empty match at the same position forever and the loop would never end.

Invalid patterns are caught with try/catch and the error message is shown instead of breaking the page.