diff --git a/b2/subex.c b/b2/subex.c
index c608674..b9589c6 100644
--- a/b2/subex.c
+++ b/b2/subex.c
@@ -20,7 +20,6 @@
 
 #include "util.h"
 #include "vstring.h"
-#include "lang.h"
 #include "relop.h"
 #include "subst.h"
 #include "subex.h"
@@ -106,21 +105,18 @@ static char *compose(const struct chunk *c,
 			val = var_lookup(in, c->u.var);
 			if (!val)
 				val = var_lookup(out, c->u.var);
-			if (!val)
-				yyerrorf("unknown variable \"%s\"", c->u.var);
+			assert(val);
 			append(&res, &res_len, val);
 			break;
 		case ct_sub:
 			n = c->u.sub;
-			if (!s)
-				yyerrorf("$%c without prior match",
-				    n ? n+'0' : '$');
+			assert(s);
 			if (!n) {
 				append(&res, &res_len, s);
 				break;
 			}
 			if (match[n-1].rm_so == -1)
-				yyerrorf("substring $%d out of range", n);
+				break;
 			len = match[n-1].rm_eo-match[n-1].rm_so;
 			tmp = alloc_size(len);
 			memcpy(tmp, s+match[n-1].rm_so, len);
diff --git a/b2/subst.c b/b2/subst.c
index 7415419..6cf2058 100644
--- a/b2/subst.c
+++ b/b2/subst.c
@@ -110,11 +110,11 @@ struct subst *subst_match(const char *src, const char *re)
 	char error[1000];
 	struct subst *sub;
 	char *tmp;
-	int parens, err;
+	int err;
 
 	sub = alloc_subst(st_match);
 	sub->u.match.src = src;
-	tmp = prepare_re(re, &parens, sub->u.match.units);
+	tmp = prepare_re(re, &sub->u.match.parens, sub->u.match.units);
 	err = regcomp(&sub->u.match.re, tmp, REG_EXTENDED);
 	free(tmp);
 	if (err) {
@@ -287,18 +287,74 @@ static const struct subst *resolve_jump(const char *name,
 }
 
 
-static void recurse_fin(struct subst *sub, const struct parent *parent)
+static int find_var_use(const char *var, const struct subst *sub)
 {
-	struct parent next = {
-		.sub = sub,
-		.parent = parent,
-	};
-
 	while (sub) {
 		switch (sub->type) {
 		case st_match:
-			recurse_fin(sub->u.match.block, &next);
+			if (sub->u.match.src == var)
+				return 1;
+			break;
 		case st_assign:
+			if (sub->u.assign.dst == var)
+				return 1;
+			break;
+		default:
+			break;
+		}
+		sub = sub->prev;
+	}
+	return 0;
+}
+
+
+static void check_chunks(const struct chunk *c, const struct parent *parent,
+    const struct subst *prev)
+{
+	int parens;
+
+	while (c) {
+		switch (c->type) {
+		case ct_sub:
+			if (!parent)
+				yyerrorf("$%c without match",
+				    c->u.sub ? c->u.sub+'0' : '$');
+			parens = parent->sub->u.match.parens;
+			if (c->u.sub > parens)
+				yyerrorf("$%d but only %d parenthes%s",
+				    c->u.sub, parens,
+				    parens == 1 ? "is" : "es");
+			break;
+		case ct_var:
+			if (!find_var_use(c->u.var, prev))
+				yyerrorf("$%s may be undefined", c->u.var);
+			break;
+		default:
+			break;
+		}
+		c = c->next;
+	}
+}
+
+
+static void recurse_fin(struct subst *sub, const struct parent *parent)
+{
+	struct parent next = {
+		.parent = parent,
+	};
+	const struct subst *prev;
+
+	prev = parent ? parent->sub : NULL;
+	while (sub) {
+		sub->prev = prev;
+		switch (sub->type) {
+		case st_match:
+			next.sub = sub;
+			recurse_fin(sub->u.match.block, &next);
+			break;
+		case st_assign:
+			check_chunks(sub->u.assign.pat, parent, prev);
+			break;
 		case st_end:
 			break;
 		case st_break:
@@ -308,6 +364,7 @@ static void recurse_fin(struct subst *sub, const struct parent *parent)
 		default:
 			abort();
 		}
+		prev = sub;
 		sub = sub->next;
 	}
 }
diff --git a/b2/subst.h b/b2/subst.h
index f2ca3ad..bb68445 100644
--- a/b2/subst.h
+++ b/b2/subst.h
@@ -52,6 +52,7 @@ struct subst {
 			regex_t re;
 			struct subst *block;
 			char units[10];
+			int parens; /* number of parentheses */
 		} match;
 		struct {
 			const char *dst;
@@ -61,6 +62,7 @@ struct subst {
 		const struct subst *jump;
 		const char *tmp; /* jump target name; for subst_finalize */
 	} u;
+	const struct subst *prev; /* for tracking availability of variables */
 	struct subst *next;
 };