diff --git a/b2/boom.c b/b2/boom.c
index 68c5239..52d0ad9 100644
--- a/b2/boom.c
+++ b/b2/boom.c
@@ -75,6 +75,15 @@ static void do_substitutions(void)
 }
 
 
+static void parse_re(const char *re)
+{
+	char *res;
+
+	subst_match(NULL, re, &res);
+	printf("%s\n", res);
+}
+
+
 static void dump_hierarchy(struct action act)
 {
 	fields_dump(stderr, act.fields);
@@ -124,6 +133,7 @@ static void usage(const char *name)
 "  -N name           for the next file, override the name in diagnostics\n"
 "  -q var=value ...  run substitutions with the specified inputs\n"
 "  -Q var=value ...  run substitutions and then do parametric search\n"
+"  -R regex          parse and print regular expression\n"
     , name);
 	exit(1);
 }
@@ -178,6 +188,11 @@ int main(int argc, char **argv)
 			process = add_var;
 			query = 1;
 			select_parts = 1;
+		} else if (!strcmp(argv[i], "-R")) {
+			i++;
+			if (!argv[i])
+				usage(*argv);
+			parse_re(argv[i]);
 		} else
 			usage(*argv);
 	}
diff --git a/b2/lang.y b/b2/lang.y
index 0ae909a..92f7fec 100644
--- a/b2/lang.y
+++ b/b2/lang.y
@@ -619,7 +619,7 @@ block:
 			if ($4) {
 				if ($2 != rel_eq)
 					yyerror("only = allow for matching");
-				$$ = subst_match($1, $3);
+				$$ = subst_match($1, $3, NULL);
 				$$->u.match.block = $4;
 			} else {
 				$$ = subst_assign($1, $2, $3);
diff --git a/b2/subst.c b/b2/subst.c
index a6f10db..f4e063c 100644
--- a/b2/subst.c
+++ b/b2/subst.c
@@ -113,7 +113,7 @@ static char *prepare_re(const char *re, char *units)
 }
 
 
-struct subst *subst_match(const char *src, const char *re)
+struct subst *subst_match(const char *src, const char *re, char **res)
 {
 	char error[1000];
 	struct subst *sub;
@@ -124,7 +124,10 @@ struct subst *subst_match(const char *src, const char *re)
 	sub->u.match.src = src;
 	tmp = prepare_re(re, sub->u.match.units);
 	err = regcomp(&sub->u.match.re, tmp, REG_EXTENDED);
-	free(tmp);
+	if (res)
+		*res = tmp;
+	else
+		free(tmp);
 	if (err) {
 		regerror(err, &sub->u.match.re, error, sizeof(error));
 		yyerrorf("%s", error);
diff --git a/b2/subst.h b/b2/subst.h
index 0406df3..bede5c6 100644
--- a/b2/subst.h
+++ b/b2/subst.h
@@ -76,7 +76,7 @@ struct subst {
 extern const char *fn;
 
 
-struct subst *subst_match(const char *src, const char *re);
+struct subst *subst_match(const char *src, const char *re, char **res);
 struct subst *subst_assign(const char *dst, enum relop op, const char *pat);
 struct subst *subst_end(void);
 struct subst *subst_ignore(void);