fix(v4): handle multi-digit exponents in floatSafeRemainder (#5687)

* fix(v4): handle multi-digit exponents in floatSafeRemainder

The regex `/\d?e-\d?/` only matches single-digit exponents, causing
`multipleOf` validation to fail for steps like `1e-10` or smaller.

For example, with `step = 1e-10`:
- The regex matched `"1e-1"` instead of `"1e-10"`
- Captured exponent was `"1"` instead of `"10"`
- This caused incorrect decimal count, leading to NaN remainder

Changed `\d?` to `\d+` to capture multi-digit exponents correctly.

Bug discovered by whiterose (https://github.com/shakecodeslikecray/whiterose)

* test: add regression test for multipleOf with scientific notation

Ensures multipleOf correctly handles steps with multi-digit exponents
like 1e-10 and 1e-15.

This test would fail before the regex fix in floatSafeRemainder.

Author: shakecodeslikecray

packages/zod/src/v4/classic/tests/number.test.ts

@@ -153,6 +153,22 @@ test(".multipleOf() with negative divisor", () => {
   expect(() => schema.parse(7.5)).toThrow();
 });
 
+test(".multipleOf() with scientific notation (multi-digit exponents)", () => {
+  // Regression test for https://github.com/colinhacks/zod/pull/5687
+  // The regex was using \d? which only matches single-digit exponents
+  const schema = z.number().multipleOf(1e-10);
+
+  // These should all pass - they are valid multiples of 1e-10
+  expect(schema.parse(1e-10)).toEqual(1e-10);
+  expect(schema.parse(5e-10)).toEqual(5e-10);
+  expect(schema.parse(1e-9)).toEqual(1e-9); // 10 * 1e-10
+
+  // Test with 1e-15 (exponent = 15, two digits)
+  const schema15 = z.number().multipleOf(1e-15);
+  expect(schema15.parse(1e-15)).toEqual(1e-15);
+  expect(schema15.parse(3e-15)).toEqual(3e-15);
+});
+
 test(".step() validation", () => {
   const schemaPointOne = z.number().step(0.1);
   const schemaPointZeroZeroZeroOne = z.number().step(0.0001);

packages/zod/src/v4/core/util.ts

@@ -248,8 +248,8 @@ export function floatSafeRemainder(val: number, step: number): number {
   const valDecCount = (val.toString().split(".")[1] || "").length;
   const stepString = step.toString();
   let stepDecCount = (stepString.split(".")[1] || "").length;
-  if (stepDecCount === 0 && /\d?e-\d?/.test(stepString)) {
-    const match = stepString.match(/\d?e-(\d?)/);
+  if (stepDecCount === 0 && /\d?e-\d+/.test(stepString)) {
+    const match = stepString.match(/\d?e-(\d+)/);
     if (match?.[1]) {
       stepDecCount = Number.parseInt(match[1]);
     }