blixt · September 8, 2025 20:17 · Sep 8, 2025
diff --git a/findAndReplace.ts b/findAndReplace.ts
@@ -0,0 +1,48 @@
+/**
+ * Replaces old_string with new_string in the given code.
+ * Validates that old_string appears exactly once to avoid ambiguous replacements.
+ */
+export function findAndReplace(
+    code: string,
+    old_string: string,
+    new_string: string,
+): { success: true; result: string } | { success: false; error: string } {
+    let pattern = old_string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+    let occurrences = code.match(new RegExp(pattern, "g"))?.length ?? 0;
+    let searchValue: string | RegExp = old_string;
+
+    if (occurrences === 0) {
+        // Try one more time but with flexible indentation. If our more general search pattern matches exactly one place
+        // in the code, we will use it to replace the matching code.
+        //
+        // Note: We replace with [\t ] instead of \s here because otherwise it would also match the preceding newline.
+        pattern = pattern.replace(/^\s+/gm, "([\\t ]+)");
+        searchValue = new RegExp(pattern, "g");
+        occurrences = code.match(searchValue)?.length ?? 0;
+        if (occurrences === 0) {
+            return {
+                success: false,
+                error: `The string "${old_string}" was not found in the code.`,
+            };
+        }
+        if (occurrences > 1) {
+            return {
+                success: false,
+                error: `The string "${old_string}" does not appear in the code. However, a similar string with different indentation exists. Please add more context and use exact indentation to avoid ambiguous replacements.`,
+            };
+        }
+    }
+
+    if (occurrences > 1) {
+        return {
+            success: false,
+            error: `The string "${old_string}" appears ${occurrences} times in the code. Please be more specific to avoid ambiguous replacements.`,
+        };
+    }
+
+    return {
+        success: true,
+        // Note: Using a function here also avoids the default behavior of replace changing "$$" to "$".
+        result: code.replace(searchValue, (_, indent) => `${typeof indent === "string" ? indent : ""}${new_string}`),
+    };
+}
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