regexp already merges adjacent overlapping ranges.  this patch
also merges adjacent, non-overlapping ranges..  for example [ab] -> [a-b].
this patch is built on the changes in regexpclass.  the actual change
is 2 lines plus commented-out debugging.

this patch is a significant performance win for some cases:
/tmp/sed is the current sources version.

; time /tmp/sed -n '/[α-ω]/p' </tmp/gs>/dev/null
1.10u 0.02s 1.17r 	 /tmp/sed -n /[α-ω]/p
; time sed -n '/[α-ω]/p' </tmp/gs>/dev/null
1.09u 0.02s 1.17r 	 sed -n /[α-ω]/p

; time /tmp/sed -n '/[αβγδεζηθικλμνξοπρςστυφχψω]/p' </tmp/gs>/dev/null
1.72u 0.02s 1.81r 	 /tmp/sed -n /[αβγδεζηθικλμνξοπρςστυφχψω]/p
; time /bin/sed -n '/[αβγδεζηθικλμνξοπρςστυφχψω]/p' </tmp/gs>/dev/null
1.10u 0.02s 1.17r 	 /bin/sed -n /[αβγδεζηθικλμνξοπρςστυφχψω]/p

debugging output (in the code, commented out)
# before change
; echo x | 8.xsed -n '/[abcdefghijklmnopqrstuvwxyz]/p' 
nspan = 26
a a	0061 0061
b b	0062 0062
c c	0063 0063
d d	0064 0064
e e	0065 0065
f f	0066 0066
g g	0067 0067
h h	0068 0068
i i	0069 0069
j j	006a 006a
k k	006b 006b
l l	006c 006c
m m	006d 006d
n n	006e 006e
o o	006f 006f
p p	0070 0070
q q	0071 0071
r r	0072 0072
s s	0073 0073
t t	0074 0074
u u	0075 0075
v v	0076 0076
w w	0077 0077
x x	0078 0078
y y	0079 0079
z z	007a 007a
x
# after change
; ; echo x | 8.xsed -n '/[abcdefghijklmnopqrstuvwxyz]/p' 
nspan = 1
a z	0061 007a
x

Reference: /n/sources/patch/applied/regexpcmerge
Date: Tue Dec  1 03:23:54 CET 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libregexp/regcomp.c	Tue Dec  1 03:19:00 2009
+++ /sys/src/libregexp/regcomp.c	Tue Dec  1 03:18:58 2009
@@ -383,11 +383,26 @@
 	return RUNE;
 }
 
+static void
+debugspan(void)
+{
+#ifdef DEBUG
+	int i, nspan;
+	Rune r;
+
+	nspan = yyclassp->end - yyclassp->spans >>1;
+	fprint(2, "nspan = %d\n", nspan);
+	p = yyclassp->spans;
+	for(i = 0; i < nspan; i++)
+		print("%C %C	%.4ux %.4ux\n", p[2*i], p[2*i+1], p[2*i], p[2*i+1]);
+#endif
+}
+
 static int
 bldcclass(void)
 {
 	int type;
-	Rune r[NCCRUNE];
+	Rune r[NSPANS*2];
 	Rune *p, *ep, *np;
 	Rune rune;
 	int quoted;
@@ -408,7 +423,11 @@
 	}
 
 	/* parse class into a set of spans */
-	for(; ep<&r[NCCRUNE];){
+	for(;;){
+		if(ep == r + nelem(r)){
+			rcerror("class too large");
+			return 0;
+		}
 		if(rune == 0){
 			rcerror("malformed '[]'");
 			return 0;
@@ -455,8 +474,8 @@
 		np[0] = *p++;
 		np[1] = *p++;
 		for(; p < ep; p += 2)
-			if(p[0] <= np[1]){
-				if(p[1] > np[1])
+			if(p[0] <= np[1]+1){
+				if(p[1] >= np[1])
 					np[1] = p[1];
 			} else {
 				np += 2;
@@ -464,6 +483,7 @@
 				np[1] = p[1];
 			}
 		yyclassp->end = np+2;
+		debugspan();
 	}
 
 	return type;