libc/regex: const'r'us

[Miod Vallat]

Spencer's code was written before const was a thing, but we can do
better. Neither regcomp(3) nor regex(3) modify the strings they are
being passed, so we can keep internal pointers as const as well and
avoid {dub,spur}ious casts.

While there, the temporary array in nonnewline() can be made static
const as well rather than recreated every time.

Index: regcomp.c
===================================================================
RCS file: /OpenBSD/src/lib/libc/regex/regcomp.c,v
retrieving revision 1.38
diff -u -p -r1.38 regcomp.c
--- regcomp.c   30 Dec 2020 08:59:17 -0000      1.38
+++ regcomp.c   31 Dec 2020 07:21:31 -0000
@@ -53,8 +53,8 @@
  * other clumsinesses
  */
 struct parse {
-       char *next;             /* next character in RE */
-       char *end;              /* end of string (-> NUL normally) */
+       const char *next;       /* next character in RE */
+       const char *end;        /* end of string (-> NUL normally) */
        int error;              /* has an error been seen? */
        sop *strip;             /* malloced strip */
        sopno ssize;            /* malloced strip size (allocated) */
@@ -179,7 +178,7 @@ regcomp(regex_t *preg, const char *patte
 
        /* set things up */
        p->g = g;
-       p->next = (char *)pattern;      /* convenience; we do not modify it */
+       p->next = pattern;
        p->end = p->next + len;
        p->error = 0;
        p->ncsalloc = 0;
@@ -754,7 +749,7 @@ p_b_term(struct parse *p, cset *cs)
 static void
 p_b_cclass(struct parse *p, cset *cs)
 {
-       char *sp = p->next;
+       const char *sp = p->next;
        const struct cclass *cp;
        size_t len;
        const char *u;
@@ -816,7 +811,7 @@ static char                 /* value of collating elem
 p_b_coll_elem(struct parse *p,
     int endc)                  /* name ended by endc,']' */
 {
-       char *sp = p->next;
+       const char *sp = p->next;
        const struct cname *cp;
        size_t len;
 
@@ -860,8 +855,8 @@ othercase(int ch)
 static void
 bothcases(struct parse *p, int ch)
 {
-       char *oldnext = p->next;
-       char *oldend = p->end;
+       const char *oldnext = p->next;
+       const char *oldend = p->end;
        char bracket[3];
 
        ch = (uch)ch;
@@ -921,16 +911,12 @@ backslash(struct parse *p, int ch)
 static void
 nonnewline(struct parse *p)
 {
-       char *oldnext = p->next;
-       char *oldend = p->end;
-       char bracket[4];
+       const char *oldnext = p->next;
+       const char *oldend = p->end;
+       static const char bracket[4] = { '^', '\n', ']', '\0' };
 
        p->next = bracket;
        p->end = bracket+3;
-       bracket[0] = '^';
-       bracket[1] = '\n';
-       bracket[2] = ']';
-       bracket[3] = '\0';
        p_bracket(p);
        assert(p->next == bracket+3);
        p->next = oldnext;