diff --git a/src/normal.c b/src/normal.c
index 74a5efe..fe1a182 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -5477,6 +5477,14 @@
 #ifdef FEAT_SYN_HL
 	/* Clear all syntax states to force resyncing. */
 	syn_stack_free_all(curwin->w_s);
+# ifdef FEAT_RELTIME
+	{
+	    win_T *wp;
+
+	    FOR_ALL_WINDOWS(wp)
+		wp->w_s->b_syn_slow = FALSE;
+	}
+# endif
 #endif
 	redraw_later(CLEAR);
     }
diff --git a/src/proto/syntax.pro b/src/proto/syntax.pro
index aae187f..a505ee7 100644
--- a/src/proto/syntax.pro
+++ b/src/proto/syntax.pro
@@ -1,5 +1,5 @@
 /* syntax.c */
-void syntax_start(win_T *wp, linenr_T lnum);
+void syntax_start(win_T *wp, linenr_T lnum, proftime_T *syntax_tm);
 void syn_stack_free_all(synblock_T *block);
 void syn_stack_apply_changes(buf_T *buf);
 void syntax_end_parsing(linenr_T lnum);
diff --git a/src/regexp.c b/src/regexp.c
index e13061f..c4745ce 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -5756,8 +5756,6 @@
 	    printf("Premature EOL\n");
 #endif
 	}
-	if (status == RA_FAIL)
-	    got_int = TRUE;
 	return (status == RA_MATCH);
     }
 
@@ -8224,8 +8222,6 @@
 }
 #endif
 
-static int vim_regexec_both(regmatch_T *rmp, char_u *line, colnr_T col, int nl);
-
 /*
  * Match a regexp against a string.
  * "rmp->regprog" is a compiled regexp as returned by vim_regcomp().
@@ -8236,7 +8232,7 @@
  * Return TRUE if there is a match, FALSE if not.
  */
     static int
-vim_regexec_both(
+vim_regexec_string(
     regmatch_T	*rmp,
     char_u	*line,  /* string to match against */
     colnr_T	col,    /* column to start looking for match */
@@ -8299,12 +8295,12 @@
     char_u	*line,
     colnr_T	col)
 {
-    int r;
-    regmatch_T regmatch;
+    int		r;
+    regmatch_T	regmatch;
 
     regmatch.regprog = *prog;
     regmatch.rm_ic = ignore_case;
-    r = vim_regexec_both(&regmatch, line, col, FALSE);
+    r = vim_regexec_string(&regmatch, line, col, FALSE);
     *prog = regmatch.regprog;
     return r;
 }
@@ -8316,7 +8312,7 @@
     int
 vim_regexec(regmatch_T *rmp, char_u *line, colnr_T col)
 {
-    return vim_regexec_both(rmp, line, col, FALSE);
+    return vim_regexec_string(rmp, line, col, FALSE);
 }
 
 #if defined(FEAT_MODIFY_FNAME) || defined(FEAT_EVAL) \
@@ -8329,7 +8325,7 @@
     int
 vim_regexec_nl(regmatch_T *rmp, char_u *line, colnr_T col)
 {
-    return vim_regexec_both(rmp, line, col, TRUE);
+    return vim_regexec_string(rmp, line, col, TRUE);
 }
 #endif
 
diff --git a/src/screen.c b/src/screen.c
index c6d8f0e..617051c 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -124,7 +124,7 @@
 static void fill_foldcolumn(char_u *p, win_T *wp, int closed, linenr_T lnum);
 static void copy_text_attr(int off, char_u *buf, int len, int attr);
 #endif
-static int win_line(win_T *, linenr_T, int, int, int nochange);
+static int win_line(win_T *, linenr_T, int, int, int nochange, proftime_T *syntax_tm);
 static int char_needs_redraw(int off_from, int off_to, int cols);
 #ifdef FEAT_RIGHTLEFT
 static void screen_line(int row, int coloff, int endcol, int clear_width, int rlflag);
@@ -185,6 +185,11 @@
 static int screen_char_attr = 0;
 #endif
 
+#if defined(FEAT_SYN_HL) && defined(FEAT_RELTIME)
+/* Can limit syntax highlight time to 'redrawtime'. */
+# define SYN_TIME_LIMIT 1
+#endif
+
 /*
  * Redraw the current window later, with update_screen(type).
  * Set must_redraw only if not already set to a higher value.
@@ -923,6 +928,9 @@
 {
     int		row;
     int		j;
+#ifdef SYN_TIME_LIMIT
+    proftime_T	syntax_tm;
+#endif
 
     /* Don't do anything if the screen structures are (not yet) valid. */
     if (!screen_valid(TRUE) || updating_screen)
@@ -931,6 +939,10 @@
     if (lnum >= wp->w_topline && lnum < wp->w_botline
 				 && foldedCount(wp, lnum, &win_foldinfo) == 0)
     {
+#ifdef SYN_TIME_LIMIT
+	/* Set the time limit to 'redrawtime'. */
+	profile_setlimit(p_rdt, &syntax_tm);
+#endif
 	update_prepare();
 
 	row = 0;
@@ -944,7 +956,13 @@
 		start_search_hl();
 		prepare_search_hl(wp, lnum);
 # endif
-		win_line(wp, lnum, row, row + wp->w_lines[j].wl_size, FALSE);
+		win_line(wp, lnum, row, row + wp->w_lines[j].wl_size, FALSE,
+#ifdef SYN_TIME_LIMIT
+			&syntax_tm
+#else
+			NULL
+#endif
+			);
 # if defined(FEAT_SEARCH_EXTRA)
 		end_search_hl();
 # endif
@@ -1140,6 +1158,9 @@
 #if defined(FEAT_SYN_HL) || defined(FEAT_SEARCH_EXTRA)
     int		save_got_int;
 #endif
+#ifdef SYN_TIME_LIMIT
+    proftime_T	syntax_tm;
+#endif
 
     type = wp->w_redr_type;
 
@@ -1792,6 +1813,10 @@
     save_got_int = got_int;
     got_int = 0;
 #endif
+#ifdef SYN_TIME_LIMIT
+    /* Set the time limit to 'redrawtime'. */
+    profile_setlimit(p_rdt, &syntax_tm);
+#endif
 #ifdef FEAT_FOLDING
     win_foldinfo.fi_level = 0;
 #endif
@@ -2086,7 +2111,13 @@
 		/*
 		 * Display one line.
 		 */
-		row = win_line(wp, lnum, srow, wp->w_height, mod_top == 0);
+		row = win_line(wp, lnum, srow, wp->w_height, mod_top == 0,
+#ifdef SYN_TIME_LIMIT
+			&syntax_tm
+#else
+			NULL
+#endif
+			);
 
 #ifdef FEAT_FOLDING
 		wp->w_lines[idx].wl_folded = FALSE;
@@ -2957,7 +2988,8 @@
     linenr_T	lnum,
     int		startrow,
     int		endrow,
-    int		nochange UNUSED)	/* not updating for changed text */
+    int		nochange UNUSED,	/* not updating for changed text */
+    proftime_T	*syntax_tm)
 {
     int		col = 0;		/* visual column on screen */
     unsigned	off;			/* offset in ScreenLines/ScreenAttrs */
@@ -3158,20 +3190,29 @@
     extra_check = 0;
 #endif
 #ifdef FEAT_SYN_HL
-    if (syntax_present(wp) && !wp->w_s->b_syn_error)
+    if (syntax_present(wp) && !wp->w_s->b_syn_error
+# ifdef SYN_TIME_LIMIT
+	    && !wp->w_s->b_syn_slow
+# endif
+       )
     {
 	/* Prepare for syntax highlighting in this line.  When there is an
 	 * error, stop syntax highlighting. */
 	save_did_emsg = did_emsg;
 	did_emsg = FALSE;
-	syntax_start(wp, lnum);
+	syntax_start(wp, lnum, syntax_tm);
 	if (did_emsg)
 	    wp->w_s->b_syn_error = TRUE;
 	else
 	{
 	    did_emsg = save_did_emsg;
-	    has_syntax = TRUE;
-	    extra_check = TRUE;
+#ifdef SYN_TIME_LIMIT
+	    if (!wp->w_s->b_syn_slow)
+#endif
+	    {
+		has_syntax = TRUE;
+		extra_check = TRUE;
+	    }
 	}
     }
 
@@ -3548,7 +3589,7 @@
 # ifdef FEAT_SYN_HL
 	    /* Need to restart syntax highlighting for this line. */
 	    if (has_syntax)
-		syntax_start(wp, lnum);
+		syntax_start(wp, lnum, syntax_tm);
 # endif
 	}
 #endif
@@ -4491,6 +4532,10 @@
 		    }
 		    else
 			did_emsg = save_did_emsg;
+#ifdef SYN_TIME_LIMIT
+		    if (wp->w_s->b_syn_slow)
+			has_syntax = FALSE;
+#endif
 
 		    /* Need to get the line again, a multi-line regexp may
 		     * have made it invalid. */
diff --git a/src/structs.h b/src/structs.h
index d79b225..c7aa8f2 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1797,6 +1797,9 @@
     hashtab_T	b_keywtab;		/* syntax keywords hash table */
     hashtab_T	b_keywtab_ic;		/* idem, ignore case */
     int		b_syn_error;		/* TRUE when error occurred in HL */
+# ifdef FEAT_RELTIME
+    int		b_syn_slow;		/* TRUE when 'redrawtime' reached */
+# endif
     int		b_syn_ic;		/* ignore case for :syn cmds */
     int		b_syn_spell;		/* SYNSPL_ values */
     garray_T	b_syn_patterns;		/* table for syntax patterns */
diff --git a/src/syntax.c b/src/syntax.c
index a7b3084..6aec63d 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -367,6 +367,9 @@
 static win_T	*syn_win;		/* current window for highlighting */
 static buf_T	*syn_buf;		/* current buffer for highlighting */
 static synblock_T *syn_block;		/* current buffer for highlighting */
+#ifdef FEAT_RELTIME
+static proftime_T *syn_tm;
+#endif
 static linenr_T current_lnum = 0;	/* lnum of current state */
 static colnr_T	current_col = 0;	/* column of current state */
 static int	current_state_stored = 0; /* TRUE if stored current state
@@ -494,7 +497,7 @@
  * window.
  */
     void
-syntax_start(win_T *wp, linenr_T lnum)
+syntax_start(win_T *wp, linenr_T lnum, proftime_T *syntax_tm UNUSED)
 {
     synstate_T	*p;
     synstate_T	*last_valid = NULL;
@@ -524,6 +527,9 @@
     }
     changedtick = CHANGEDTICK(syn_buf);
     syn_win = wp;
+#ifdef FEAT_RELTIME
+    syn_tm = syntax_tm;
+#endif
 
     /*
      * Allocate syntax stack when needed.
@@ -3295,6 +3301,9 @@
     syn_time_T  *st UNUSED)
 {
     int r;
+#ifdef FEAT_RELTIME
+    int timed_out = FALSE;
+#endif
 #ifdef FEAT_PROFILE
     proftime_T	pt;
 
@@ -3303,7 +3312,13 @@
 #endif
 
     rmp->rmm_maxcol = syn_buf->b_p_smc;
-    r = vim_regexec_multi(rmp, syn_win, syn_buf, lnum, col, NULL, NULL);
+    r = vim_regexec_multi(rmp, syn_win, syn_buf, lnum, col,
+#ifdef FEAT_RELTIME
+	    syn_tm, &timed_out
+#else
+	    NULL, NULL
+#endif
+	    );
 
 #ifdef FEAT_PROFILE
     if (syn_time_on)
@@ -3317,6 +3332,10 @@
 	    ++st->match;
     }
 #endif
+#ifdef FEAT_RELTIME
+    if (timed_out)
+	syn_win->w_s->b_syn_slow = TRUE;
+#endif
 
     if (r > 0)
     {
@@ -3575,6 +3594,9 @@
     int i;
 
     block->b_syn_error = FALSE;	    /* clear previous error */
+#ifdef FEAT_RELTIME
+    block->b_syn_slow = FALSE;	    /* clear previous timeout */
+#endif
     block->b_syn_ic = FALSE;	    /* Use case, by default */
     block->b_syn_spell = SYNSPL_DEFAULT; /* default spell checking */
     block->b_syn_containedin = FALSE;
@@ -6542,7 +6564,7 @@
     if (wp->w_buffer != syn_buf
 	    || lnum != current_lnum
 	    || col < current_col)
-	syntax_start(wp, lnum);
+	syntax_start(wp, lnum, NULL);
     else if (wp->w_buffer == syn_buf
 	    && lnum == current_lnum
 	    && col > current_col)
@@ -6611,9 +6633,14 @@
     int		i;
 
     /* Return quickly when there are no fold items at all. */
-    if (wp->w_s->b_syn_folditems != 0)
+    if (wp->w_s->b_syn_folditems != 0
+	    && !wp->w_s->b_syn_error
+# ifdef SYN_TIME_LIMIT
+	    && !wp->w_s->b_syn_slow
+# endif
+	    )
     {
-	syntax_start(wp, lnum);
+	syntax_start(wp, lnum, NULL);
 
 	for (i = 0; i < current_state.ga_len; ++i)
 	    if (CUR_STATE(i).si_flags & HL_FOLD)
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index eb093c8..427ed7b 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -424,3 +424,37 @@
   hi Normal ctermbg=12
   call assert_equal('dark', &bg)
 endfunc
+
+func Test_syntax_hangs()
+  if !has('reltime') || !has('float') || !has('syntax')
+    return
+  endif
+
+  " This pattern takes a long time to match, it should timeout.
+  new
+  call setline(1, ['aaa', repeat('abc ', 1000), 'ccc'])
+  let start = reltime()
+  set nolazyredraw redrawtime=101
+  syn match Error /\%#=1a*.*X\@<=b*/
+  redraw
+  let elapsed = reltimefloat(reltime(start))
+  call assert_true(elapsed > 0.1)
+  call assert_true(elapsed < 1.0)
+
+  " second time syntax HL is disabled
+  let start = reltime()
+  redraw
+  let elapsed = reltimefloat(reltime(start))
+  call assert_true(elapsed < 0.1)
+
+  " after CTRL-L the timeout flag is reset
+  let start = reltime()
+  exe "normal \<C-L>"
+  redraw
+  let elapsed = reltimefloat(reltime(start))
+  call assert_true(elapsed > 0.1)
+  call assert_true(elapsed < 1.0)
+
+  set redrawtime&
+  bwipe!
+endfunc
diff --git a/src/version.c b/src/version.c
index 4683ba4..59f0d09 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    647,
+/**/
     646,
 /**/
     645,