diff --git a/src/analyze.c b/src/analyze.c
index 20deee8..5ea3027 100644
--- a/src/analyze.c
+++ b/src/analyze.c
@@ -113,7 +113,7 @@ discard_confusing_lines (struct file_data filevec[])
   for (int f = 0; f < 2; f++)
     {
       lin end = filevec[f].buffered_lines;
-      register char *discards = discarded[f];
+      char *discards = discarded[f];
 
       for (lin i = 0; i < end; i++)
         {
@@ -123,8 +123,7 @@ discard_confusing_lines (struct file_data filevec[])
           else if (discards[i] != 0)
             {
               /* We have found a nonprovisional discard.  */
-              register lin j;
-              lin provisional = 0;
+              lin provisional = 0, j;
 
               /* Find end of this run of discardable lines.
                  Count how many are provisionally discardable.  */
diff --git a/src/diff3.c b/src/diff3.c
index 301d4c9..dca7680 100644
--- a/src/diff3.c
+++ b/src/diff3.c
@@ -840,8 +840,8 @@ copy_stringlist (char *const fromptrs[], idx_t const fromlengths[],
                  char *toptrs[], idx_t tolengths[],
                  lin copynum)
 {
-  register char *const *f = fromptrs;
-  register char **t = toptrs;
+  char *const *f = fromptrs;
+  char **t = toptrs;
   idx_t const *fl = fromlengths;
   idx_t *tl = tolengths;
 
diff --git a/src/dir.c b/src/dir.c
index a62539a..5a69391 100644
--- a/src/dir.c
+++ b/src/dir.c
@@ -64,7 +64,7 @@ dir_read (struct file_data const *dir, struct dirdata *dirdata)
   if (dir->desc != -1)
     {
       /* Open the directory and check for errors.  */
-      register DIR *reading = opendir (dir->name);
+      DIR *reading = opendir (dir->name);
       if (!reading)
         return false;
 
diff --git a/src/ifdef.c b/src/ifdef.c
index ca24219..ba72676 100644
--- a/src/ifdef.c
+++ b/src/ifdef.c
@@ -110,10 +110,10 @@ format_ifdef (char const *format, lin beg0, lin end0, lin beg1, lin end1)
    If OUT is zero, do not actually print anything; just scan the format.  */
 
 static char const *
-format_group (register FILE *out, char const *format, char endchar,
+format_group (FILE *out, char const *format, char endchar,
               struct group const *groups)
 {
-  register char const *f = format;
+  char const *f = format;
 
   for (char c; (c = *f) != endchar && c != 0; )
     {
@@ -225,7 +225,7 @@ groups_letter_value (struct group const *g, char letter)
 /* Print to file OUT, using FORMAT to print the line group GROUP.
    But do nothing if OUT is zero.  */
 static void
-print_ifdef_lines (register FILE *out, char const *format,
+print_ifdef_lines (FILE *out, char const *format,
                    struct group const *group)
 {
   if (!out)
@@ -255,8 +255,8 @@ print_ifdef_lines (register FILE *out, char const *format,
 
   for (;  from < upto;  from++)
     {
-      register char c;
-      register char const *f = format;
+      char c;
+      char const *f = format;
 
       while ((c = *f++) != 0)
         {
@@ -376,7 +376,7 @@ do_printf_spec (FILE *out, char const *spec,
 static char const *
 scan_char_literal (char const *lit, char *valptr)
 {
-  register char const *p = lit;
+  char const *p = lit;
   char value;
   char c = *p++;
 
diff --git a/src/side.c b/src/side.c
index 797bb23..c4042dd 100644
--- a/src/side.c
+++ b/src/side.c
@@ -75,14 +75,14 @@ print_half_line (char const *const *line, intmax_t indent, intmax_t out_bound)
   FILE *out = outfile;
   intmax_t in_position = 0;
   intmax_t out_position = 0;
-  register char const *text_pointer = line[0];
-  register char const *text_limit = line[1];
+  char const *text_pointer = line[0];
+  char const *text_limit = line[1];
   mbstate_t mbstate = { 0 };
 
   while (text_pointer < text_limit)
     {
       char const *tp0 = text_pointer;
-      register char c = *text_pointer++;
+      char c = *text_pointer++;
 
       switch (c)
         {
diff --git a/src/util.c b/src/util.c
index d5167d5..cb8f45d 100644
--- a/src/util.c
+++ b/src/util.c
@@ -1060,14 +1060,14 @@ finish_output (void)
 bool
 lines_differ (char const *s1, char const *s2)
 {
-  register char const *t1 = s1;
-  register char const *t2 = s2;
+  char const *t1 = s1;
+  char const *t2 = s2;
   intmax_t tab = 0, column = 0;
 
   while (1)
     {
-      register unsigned char c1 = *t1++;
-      register unsigned char c2 = *t2++;
+      unsigned char c1 = *t1++;
+      unsigned char c2 = *t2++;
 
       /* Test for exact char equality first, since it's a common case.  */
       if (c1 != c2)
@@ -1364,8 +1364,8 @@ output_1_line (char const *base, char const *limit, char const *flag_format,
     }
   else
     {
-      register FILE *out = outfile;
-      register char const *t = base;
+      FILE *out = outfile;
+      char const *t = base;
       intmax_t tab = 0, column = 0, tab_size = tabsize;
       int counter_proc_signals = 0;