diff --git a/re2/re2.cc b/re2/re2.cc
index ca9775e1f27a91567b699f4c46991f8e096923e0..91d9880f4d02aa56d848b80ca69d7c2769c439d0 100644
--- a/re2/re2.cc
+++ b/re2/re2.cc
@@ -1023,22 +1023,7 @@ namespace re2
   // E.g. if rewrite == "foo \\2,\\1", returns 2.
   int RE2::MaxSubmatch(const StringPiece &rewrite)
   {
-    int max = 0;
-    for (const char *s = rewrite.data(), *end = s + rewrite.size();
-         s < end; s++)
-    {
-      if (*s == '\\')
-      {
-        s++;
-        int c = (s < end) ? *s : -1;
-        if (isdigit(c))
-        {
-          int n = (c - '0');
-          if (n > max)
-            max = n;
-        }
-      }
-    }
+    int max = rure_max_submatch(rewrite.data());
     return max;
   }
 
diff --git a/regex-capi/include/rure.h b/regex-capi/include/rure.h
index 1817e25869e22d7907f5b62663f2618f2058ec11..67194866ae31bab1343f5179f1b316710b166d07 100644
--- a/regex-capi/include/rure.h
+++ b/regex-capi/include/rure.h
@@ -610,6 +610,7 @@ const char *rure_replace_all(rure *re, const uint8_t *haystack, size_t len_h,
 
 rure *rure_new(const uint8_t *pattern, size_t length);
 bool rure_consume(rure *re, const uint8_t *haystack, size_t length, rure_match *match);
+int rure_max_submatch(const char *rewrite);
 
 #ifdef __cplusplus
 }
diff --git a/regex-capi/src/rure.rs b/regex-capi/src/rure.rs
index 17e35524d4a2a7a5cc86297cdea9472a47bbc4fe..b88ffc4bfb5886823dd9af9b6b672c8373fb2ed0 100644
--- a/regex-capi/src/rure.rs
+++ b/regex-capi/src/rure.rs
@@ -721,3 +721,29 @@ ffi_fn! {
     }
 
 }
+
+ffi_fn! {
+    fn rure_max_submatch(rewrite: *const c_char) -> i32 {
+        let mut max: i32 = 0;
+        let mut flag = 0;
+        let zero_number = '0' as i32;
+        let len = unsafe { CStr::from_ptr(rewrite).to_bytes().len() };
+        let pat = rewrite as *const u8;
+        let text = unsafe { slice::from_raw_parts(pat, len) };
+        let rewrite = std::str::from_utf8(text).unwrap();
+        for s in rewrite.chars() {
+            if s == '\\' {
+                flag = 1;
+                continue;
+            }
+            if s.is_ascii_digit() && flag == 1 {
+                let max_ = s as i32 - zero_number;
+                if max_ > max {
+                    max = max_;
+                }
+                flag = 0;
+            }
+        }
+        max
+    }
+}