From 52ede3ef6f6eec32eafd48695a8a9f54074581d9 Mon Sep 17 00:00:00 2001
From: Raphael Fakhri <153192858+RaphaelFakhri@users.noreply.github.com>
Date: Tue, 27 Oct 2026 15:08:22 +0000
Subject: [PATCH] fix(fast-check): keep regex flags when generating matching
 strings

---
 ark/fast-check/__tests__/arktypeFastCheck.test.ts | 5 +++++
 ark/fast-check/arbitraries/string.ts              | 9 ++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/ark/fast-check/__tests__/arktypeFastCheck.test.ts b/ark/fast-check/__tests__/arktypeFastCheck.test.ts
index da9423fd9d..071e7deb91 100644
--- a/ark/fast-check/__tests__/arktypeFastCheck.test.ts
+++ b/ark/fast-check/__tests__/arktypeFastCheck.test.ts
@@ -89,6 +89,11 @@ contextualize(() => {
 			const arbitrary = arkToArbitrary(T)
 			assertProperty(arbitrary, T)
 		})
+		it("regex with flags", () => {
+			const T = type(/^\u{1F600}[a-z]$/iu)
+			const arbitrary = arkToArbitrary(T)
+			assertProperty(arbitrary, T)
+		})
 		it("multiple regexes", () => {
 			const T = type("string.email").and("string.alpha")
 			attest(() => arkToArbitrary(T)).throws(
diff --git a/ark/fast-check/arbitraries/string.ts b/ark/fast-check/arbitraries/string.ts
index 2c9548eef1..f7796d5660 100644
--- a/ark/fast-check/arbitraries/string.ts
+++ b/ark/fast-check/arbitraries/string.ts
@@ -11,7 +11,9 @@ export const buildStringArbitrary = (
 	if ("pattern" in stringConstraints) {
 		if (stringConstraints.minLength || stringConstraints.maxLength)
 			throwInternalError("Bounded regex is not supported.")
-		return fc.stringMatching(new RegExp(stringConstraints.pattern))
+		return fc.stringMatching(
+			new RegExp(stringConstraints.pattern, stringConstraints.flags)
+		)
 	}
 
 	return fc.string(stringConstraints)
@@ -22,6 +24,7 @@ const getFastCheckStringConstraints = (
 ) => {
 	const stringConstraints: fc.StringConstraints & {
 		pattern?: string
+		flags?: string
 	} = {}
 	for (const refinement of refinements) {
 		if (refinement.hasKind("pattern")) {
@@ -31,6 +34,10 @@ const getFastCheckStringConstraints = (
 				)
 			}
 			stringConstraints["pattern"] = refinement.rule
+			// fast-check doesn't support i, and dropping it only narrows what
+			// gets generated, so every generated string still matches
+			if (refinement.flags)
+				stringConstraints["flags"] = refinement.flags.replace("i", "")
 		} else if (refinement.hasKind("exactLength")) {
 			stringConstraints["minLength"] = refinement.rule
 			stringConstraints["maxLength"] = refinement.rule