util: group thousands in linear time

formatDecimal split digits with /(\d+?)(?=(\d{3})+(?!\d)|$)/g, whose
lookahead rescans the rest of a digit run at every position: quadratic in
the run's length (CodeQL js/polynomial-redos, high; inherited unchanged
from upstream). 20k digits took 258 ms, 200k about 25 s.

A loop now finds the same groups: within each digit run a group ends where
the digits left are a positive multiple of three, or at the end of the
string. Identical output to the pattern on 210,000 random inputs plus the
edge cases (letters around digits, short runs, negatives); 200k digits take
7 ms. The new spec case for a 60k-digit number fails on the old code.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

packages/util/src/format/formatDecimal.spec.ts

@@ -25,4 +25,21 @@ describe('formatDecimal', (): void => {
   it('formats negative numbers', (): void => {
     expect(formatDecimal('-123456')).toEqual('-123,456');
   });
+
+  // The pattern this replaced took quadratic time on a long digit run
+  // (CodeQL js/polynomial-redos): 60k digits took seconds; this is linear.
+  it('formats a very long number quickly', (): void => {
+    const start = Date.now();
+    const out = formatDecimal(`1${'0'.repeat(60_000)}`);
+
+    expect(Date.now() - start < 1000).toEqual(true);
+    expect(out.length).toEqual(60_001 + 20_000);
+    expect(out.startsWith('1,000,000')).toEqual(true);
+  });
+
+  it('keeps the grouping of the pattern it replaced', (): void => {
+    expect(formatDecimal('ab123cd')).toEqual('ab123cd');
+    expect(formatDecimal('12a3456')).toEqual('3,456');
+    expect(formatDecimal('1000000', '_')).toEqual('1_000_000');
+  });
 });

packages/util/src/format/formatDecimal.ts

@@ -1,8 +1,62 @@
 // Copyright 2017-2026 @pezkuwi/util authors & contributors
 // SPDX-License-Identifier: Apache-2.0
 
-// eslint-disable-next-line prefer-regex-literals
-const NUMBER_REGEX = new RegExp('(\\d+?)(?=(\\d{3})+(?!\\d)|$)', 'g');
+function isDigit (c: string): boolean {
+  return c >= '0' && c <= '9';
+}
+
+/**
+ * Splits the digits of value into thousands groups, exactly as the pattern
+ * /(\d+?)(?=(\d{3})+(?!\d)|$)/g matched them, without its quadratic cost
+ * on long digit runs (CodeQL js/polynomial-redos): within each run of digits,
+ * a group ends where the digits left in the run are a positive multiple of
+ * three, or at the end of the string. A run that never reaches such a point
+ * (two digits followed by a letter, say) contributes nothing.
+ */
+function groups (value: string): string[] {
+  const out: string[] = [];
+  const end = value.length;
+  let i = 0;
+
+  while (i < end) {
+    if (!isDigit(value[i])) {
+      i++;
+      continue;
+    }
+
+    let j = i;
+
+    while (j < end && isDigit(value[j])) {
+      j++;
+    }
+
+    let start = i;
+
+    while (start < j) {
+      let stop = -1;
+
+      for (let q = start + 1; q <= j; q++) {
+        const left = j - q;
+
+        if ((left > 0 && left % 3 === 0) || (left === 0 && j === end)) {
+          stop = q;
+          break;
+        }
+      }
+
+      if (stop === -1) {
+        break;
+      }
+
+      out.push(value.slice(start, stop));
+      start = stop;
+    }
+
+    i = j;
+  }
+
+  return out;
+}
 
 /**
  * @name formatDecimal
@@ -12,11 +66,9 @@ export function formatDecimal (value: string, separator = ','): string {
   // We can do this by adjusting the regx, however for the sake of clarity
   // we rather strip and re-add the negative sign in the output
   const isNegative = value[0].startsWith('-');
-  const matched = isNegative
-    ? value.substring(1).match(NUMBER_REGEX)
-    : value.match(NUMBER_REGEX);
+  const matched = groups(isNegative ? value.substring(1) : value);
 
-  return matched
+  return matched.length
     ? `${isNegative ? '-' : ''}${matched.join(separator)}`
     : value;
 }