diff -r 06ecc74a64fb sys/src/libregexp/regexec.c
--- a/sys/src/libregexp/regexec.c	Sun Feb 23 20:33:27 2020 +0000
+++ b/sys/src/libregexp/regexec.c	Thu Feb 27 14:26:58 2020 -0500
@@ -10,14 +10,15 @@
 };
 
 int
-regexec(Reprog *p, char *str, Resub *sem, int msize)
+regexecfn(Reprog *p, char *str, int (*more)(void*, char*, int), void *ctx, Resub *sem, int msize)
 {
 	RethreadQ lists[2], *clist, *nlist, *tmp;
 	Rethread *t, *next, *pool, *avail;
 	Reinst *ci;
 	Rune r;
-	char *sp, *ep, endc;
-	int i, matchgen, gen;
+	char *sp, *ep, *buf, endc;
+	int i, matchgen, gen, n;
+	vlong o;
 
 	memset(p->threads, 0, sizeof(Rethread)*p->nthr);
 	if(msize > NSUBEXPM)
@@ -39,9 +40,11 @@
 	gen = matchgen = 0;
 
 	sp = str;
+	buf = nil;
 	ep = nil;
 	endc = '\0';
-	if(sem != nil && msize > 0) {
+	o = 0;
+	if(more == nil && sem != nil && msize > 0) {
 		if(sem->sp != nil)
 			sp = sem->sp;
 		if(sem->ep != nil && *sem->ep != '\0') {
@@ -50,9 +53,28 @@
 			*sem->ep = '\0';
 		}
 	}
+	if(more != nil){
+		if((buf = malloc(BUFSZ + 1)) == nil)
+			regerror("out of memory");
+		buf[0] = 0;
+		sp = buf;
+		if(sem != nil && msize > 0){
+			sem->so = 0;
+			sem->eo = 0;
+		}
+	}
 
-	for(r = L'☺'; r != L'\0'; sp += i) {
+	for(r = L'☺'; r != L'\0'; sp += i, o += i) {
+Gotmore:
 		i = chartorune(&r, sp);
+		if(r == 0 && more != nil){
+			n = more(ctx, buf, BUFSZ);
+			if(n > 0){
+				sp = buf;
+				sp[n] = 0;
+				goto Gotmore;
+			}
+		}
 		gen++;
 		if(matchgen == 0) {
 			if(avail == nil) {
@@ -110,7 +132,7 @@
 			}
 			goto Done;
 		case OBOL:
-			if(sp == str || sp[-1] == '\n') {
+			if(o == 0 || sp[-1] == '\n') {
 				ci++;
 				goto Again;
 			}
@@ -141,8 +163,12 @@
 			ci = ci->a;
 			goto Again;
 		case OSAVE:
-			if(ci->sub < msize)
-				t->sem[ci->sub].sp = sp;
+			if(ci->sub < msize){
+				if(more != nil)
+					t->sem[ci->sub].so = o;
+				else
+					t->sem[ci->sub].sp = sp;
+			}
 			ci++;
 			goto Again;
 		case OUNSAVE:
@@ -150,12 +176,19 @@
 				matchgen = t->gen;
 				if(sem != nil && msize > 0) {
 					memcpy(sem, t->sem, sizeof(Resub)*msize);
-					sem->ep = sp;
+					if(more != nil)
+						sem->eo = o;
+					else
+						sem->ep = sp;
 				}
 				goto Done;
 			}
-			if(ci->sub < msize)
-				t->sem[ci->sub].ep = sp;
+			if(ci->sub < msize){
+				if(more != nil)
+					t->sem[ci->sub].eo = o;
+				else
+					t->sem[ci->sub].ep = sp;
+			}
 			ci++;
 			goto Again;
 		Done:
@@ -182,5 +215,19 @@
 	}
 	if(ep != nil)
 		*ep = endc;
+	if(more != nil)
+		free(buf);
 	return matchgen > 0 ? 1 : 0;
 }
+
+int
+regexec(Reprog *p, char *str, Resub *sem, int msize)
+{
+	return regexecfn(p, str, nil, nil, sem, msize);
+}
+
+int
+regexecrd(Reprog *p, int (*more)(void*, char*, int), void *ctx, Resub *sem, int msize)
+{
+	return regexecfn(p, nil, more, ctx, sem, msize);
+}
diff -r 06ecc74a64fb sys/src/libregexp/regimpl.h
--- a/sys/src/libregexp/regimpl.h	Sun Feb 23 20:33:27 2020 +0000
+++ b/sys/src/libregexp/regimpl.h	Thu Feb 27 14:26:58 2020 -0500
@@ -24,6 +24,7 @@
 	TSUB,
 
 	NSUBEXPM = 32,
+	BUFSZ = 4096,
 };
 
 typedef struct Parselex Parselex;