diff --git a/tests/test_hex.sh b/tests/test_hex.sh
index 0ad1be0..3340d10 100644
--- a/tests/test_hex.sh
+++ b/tests/test_hex.sh
@@ -3,7 +3,7 @@
 test_btohex() {
   # FIXME: Remove grep when this bug is fixed:
   # https://github.com/sysprog21/rv32emu/issues/561
-  result=$("${RV32EMU}" -d - -q tests/btohex.elf | grep -v '^\d' | "${JQ}" .x11)
+  result=$("${RV32EMU}" -d - -q tests/btohex.elf | grep -v '^[0-9]' | "${JQ}" .x11)
 
    # 16693 is 5A in ASCII (reversed from input due to little endian)
   test $? -eq 0 && test "${result}" -eq 13633
@@ -12,7 +12,7 @@ test_btohex() {
 test_tohex() {
   # FIXME: Remove grep when this bug is fixed:
   # https://github.com/sysprog21/rv32emu/issues/561
-  result=$("${RV32EMU}" -q tests/tohex.elf | grep -v '^\d')
+  result=$("${RV32EMU}" -q tests/tohex.elf | grep -v '^[0-9]')
 
   test $? -eq 0 && test "${result}" = "CAFEFEED"
 }