diff --git a/src/nvim/indent_c.c b/src/nvim/indent_c.c
index c89dde5bb3..81b165b5ef 100644
--- a/src/nvim/indent_c.c
+++ b/src/nvim/indent_c.c
@@ -37,6 +37,68 @@ typedef struct {
 } cpp_baseclass_cache_T;
 
 #include "indent_c.c.generated.h"
+
+/// Check if line[] contains a "//" comment, ignoring matches inside strings.
+/// Return MAXCOL if not, otherwise return the column.
+/// The line is scanned once (skipping strings), so this stays linear even on
+/// lines with many slashes (e.g. base64 data).
+int check_linecomment(const char *line)
+{
+  const char *p = line;  // scan from start
+  // skip Lispish one-line comments
+  if (curbuf->b_p_lisp) {
+    if (vim_strchr(p, ';') != NULL) {   // there may be comments
+      bool in_str = false;       // inside of string
+
+      while ((p = strpbrk(p, "\";")) != NULL) {
+        if (*p == '"') {
+          if (in_str) {
+            if (*(p - 1) != '\\') {             // skip escaped quote
+              in_str = false;
+            }
+          } else if (p == line || ((p - line) >= 2
+                                   // skip #\" form
+                                   && *(p - 1) != '\\' && *(p - 2) != '#')) {
+            in_str = true;
+          }
+        } else if (!in_str && ((p - line) < 2
+                               || (*(p - 1) != '\\' && *(p - 2) != '#'))
+                   && !is_pos_in_string(line, (colnr_T)(p - line))) {
+          break;                // found!
+        }
+        p++;
+      }
+    } else {
+      p = NULL;
+    }
+  } else {
+    // Scan the line once, skipping over strings, char constants and raw
+    // strings, instead of testing each '/' with is_pos_in_string() (which
+    // rescans from the start, making this quadratic on lines with many
+    // slashes).
+    for (; *p != NUL; p++) {
+      p = skip_string(p);
+      if (*p == NUL) {
+        break;
+      }
+      // Accept a double /, unless it's preceded with * and followed by
+      // *, because * / / * is an end and start of a C comment.
+      if (p[0] == '/' && p[1] == '/'
+          && (p == line || p[-1] != '*' || p[2] != '*')) {
+        break;
+      }
+    }
+    if (*p == NUL) {
+      p = NULL;
+    }
+  }
+
+  if (p == NULL) {
+    return MAXCOL;
+  }
+  return (int)(p - line);
+}
+
 // Find the start of a comment, not knowing if we are in a comment right now.
 // Search starts at w_cursor.lnum and goes backwards.
 // Return NULL when not inside a comment.
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index c4c62cfb63..b9ec196239 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -47,6 +47,7 @@
 #include "nvim/help.h"
 #include "nvim/highlight.h"
 #include "nvim/highlight_defs.h"
+#include "nvim/indent_c.h"
 #include "nvim/keycodes.h"
 #include "nvim/macros_defs.h"
 #include "nvim/mapping.h"
diff --git a/src/nvim/search.c b/src/nvim/search.c
index 6723f7844d..66cfef02c3 100644
--- a/src/nvim/search.c
+++ b/src/nvim/search.c
@@ -2068,10 +2068,11 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int64_t maxtravel)
       }
     }
 
-    // Track block comment state when FM_SKIPCOMM is set.
+    // Track block comment state when FM_SKIPCOMM is set.  Markers inside a
+    // string are not comments, so skip them while "inquote" is set.
     // Backward: '/' of end-marker enters comment; '*' of start-marker exits.
     // Forward:  '/' of start-marker enters comment; '/' of end-marker exits.
-    if (skip_comments && !comment_dir) {
+    if (skip_comments && !comment_dir && !inquote) {
       if (backwards) {
         // Guard pos.col < comment_col: don't misread '* /' at the '//'
         // position as a block-comment end-marker.
@@ -2339,56 +2340,6 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int64_t maxtravel)
   return (pos_T *)NULL;         // never found it
 }
 
-/// Check if line[] contains a / / comment.
-/// @returns MAXCOL if not, otherwise return the column.
-int check_linecomment(const char *line)
-{
-  const char *p = line;  // scan from start
-  // skip Lispish one-line comments
-  if (curbuf->b_p_lisp) {
-    if (vim_strchr(p, ';') != NULL) {   // there may be comments
-      bool in_str = false;       // inside of string
-
-      while ((p = strpbrk(p, "\";")) != NULL) {
-        if (*p == '"') {
-          if (in_str) {
-            if (*(p - 1) != '\\') {             // skip escaped quote
-              in_str = false;
-            }
-          } else if (p == line || ((p - line) >= 2
-                                   // skip #\" form
-                                   && *(p - 1) != '\\' && *(p - 2) != '#')) {
-            in_str = true;
-          }
-        } else if (!in_str && ((p - line) < 2
-                               || (*(p - 1) != '\\' && *(p - 2) != '#'))
-                   && !is_pos_in_string(line, (colnr_T)(p - line))) {
-          break;                // found!
-        }
-        p++;
-      }
-    } else {
-      p = NULL;
-    }
-  } else {
-    while ((p = vim_strchr(p, '/')) != NULL) {
-      // Accept a double /, unless it's preceded with * and followed by *,
-      // because * / / * is an end and start of a C comment.  Only
-      // accept the position if it is not inside a string.
-      if (p[1] == '/' && (p == line || p[-1] != '*' || p[2] != '*')
-          && !is_pos_in_string(line, (colnr_T)(p - line))) {
-        break;
-      }
-      p++;
-    }
-  }
-
-  if (p == NULL) {
-    return MAXCOL;
-  }
-  return (int)(p - line);
-}
-
 /// Move cursor briefly to character matching the one under the cursor.
 /// Used for Insert mode and "r" command.
 /// Show the match only if it is visible on the screen.
diff --git a/test/old/testdir/test_normal.vim b/test/old/testdir/test_normal.vim
index 0296f503a3..53fc655dc4 100644
--- a/test/old/testdir/test_normal.vim
+++ b/test/old/testdir/test_normal.vim
@@ -3972,6 +3972,44 @@ func Test_normal_percent_skip_comment()
   bwipe!
 endfunc
 
+" A "//" inside a string must not be treated as a line comment by "%".  The
+" line is scanned in a single pass, so this stays fast even on lines with many
+" slashes (e.g. base64 data).
+func Test_normal_percent_skip_comment_string()
+  new
+  setlocal comments=s1:/*,mb:*,ex:*/,://
+
+  " The "//" inside the string is not a comment, so "(" matches the real ")".
+  call setline(1, ['("a // b")'])
+  call cursor(1, 1)
+  normal %
+  call assert_equal([1, 10], [line('.'), col('.')])
+
+  " JSON-like: "{" matches the closing "}" although the string has slashes.
+  silent! %delete _
+  call setline(1, ['{', '  "k": "x//y",', '}'])
+  call cursor(1, 1)
+  normal %
+  call assert_equal([3, 1], [line('.'), col('.')])
+
+  " A "/*" inside a string must not start a block comment, so "(" still
+  " matches the real ")" after the string.
+  silent! %delete _
+  call setline(1, ['( "a /* b" )'])
+  call cursor(1, 1)
+  normal %
+  call assert_equal([1, 12], [line('.'), col('.')])
+
+  " A real /* */ block comment is still skipped: "(" matches the last ")".
+  silent! %delete _
+  call setline(1, ['( /* ) */ x )'])
+  call cursor(1, 1)
+  normal %
+  call assert_equal([1, 13], [line('.'), col('.')])
+
+  bwipe!
+endfunc
+
 " Test for << and >> commands to shift text by 'shiftwidth'
 func Test_normal_shift_rightleft()
   new