Thanks to John Cummings.

src/cmd/upas/send/regtest.c

@@ -0,0 +1,36 @@
+#include <u.h>
+#include <libc.h>
+#include <regexp.h>
+#include <bio.h>
+
+main(void)
+{
+	char *re;
+	char *line;
+	Reprog *prog;
+	char *cp;
+	Biobuf in;
+
+	Binit(&in, 0, OREAD);
+	print("re> ");
+	while(re = Brdline(&in, '\n')){
+		re[Blinelen(&in)-1] = 0;
+		if(*re == 0)
+			break;
+		prog = regcomp(re);
+		print("> ");
+		while(line = Brdline(&in, '\n')){
+			line[Blinelen(&in)-1] = 0;
+			if(cp = strchr(line, '\n'))
+				*cp = 0;
+			if(*line == 0)
+				break;
+			if(regexec(prog, line, 0))
+				print("yes\n");
+			else
+				print("no\n");
+			print("> ");
+		}
+		print("re> ");
+	}
+}