diff --git a/src/Examples.thy b/src/Examples.thy
index a1e99da..7f8f56e 100644
--- a/src/Examples.thy
+++ b/src/Examples.thy
@@ -109,4 +109,8 @@ lemma hexadecimal_execution:
 lemma incomplete_rejected: "prepare ''(if x y)'' = None"
   by eval
 
+lemma comment_tokens:
+  "lex (''(+ x; ignore ) tokens'' @ newline @ ''y) ; end'') [] [] = [''('', ''+'', ''x'', ''y'', '')'']"
+  by eval
+
 end
diff --git a/src/Frontend.thy b/src/Frontend.thy
index 85b2075..a997121 100644
--- a/src/Frontend.thy
+++ b/src/Frontend.thy
@@ -6,12 +6,18 @@ fun finish :: "string \<Rightarrow> string list \<Rightarrow> string list" where
   "finish [] out = out"
 | "finish word out = rev word # out"
 
-fun lex :: "string \<Rightarrow> string \<Rightarrow> string list \<Rightarrow> string list" where
-  "lex [] word out = rev (finish word out)"
-| "lex (c # cs) word out = (if c = CHR '' '' \<or> c = CHR 0x09 \<or> c = CHR 0x0A \<or> c = CHR 0x0D
-    then lex cs [] (finish word out)
-    else if c = CHR ''('' \<or> c = CHR '')'' then lex cs [] ([c] # finish word out)
-    else lex cs (c # word) out)"
+fun scan :: "string \<Rightarrow> string \<Rightarrow> string list \<Rightarrow> bool \<Rightarrow> string list" where
+  "scan [] word out comment = rev (finish word out)"
+| "scan (c # cs) word out comment = (if comment then
+      scan cs [] out (c \<noteq> CHR 0x0A \<and> c \<noteq> CHR 0x0D)
+    else if c = CHR '';'' then scan cs [] (finish word out) True
+    else if c = CHR '' '' \<or> c = CHR 0x09 \<or> c = CHR 0x0A \<or> c = CHR 0x0D
+    then scan cs [] (finish word out) False
+    else if c = CHR ''('' \<or> c = CHR '')'' then scan cs [] ([c] # finish word out) False
+    else scan cs (c # word) out False)"
+
+definition lex :: "string \<Rightarrow> string \<Rightarrow> string list \<Rightarrow> string list" where
+  "lex s word out = scan s word out False"
 
 fun digit :: "char \<Rightarrow> nat" where
   "digit c = (let n = nat_of_char c in
diff --git a/tests/cli.py b/tests/cli.py
index 1e3f962..f9936fb 100644
--- a/tests/cli.py
+++ b/tests/cli.py
@@ -26,6 +26,9 @@ def main():
         assert int(invoke('eval',source,'0','0','0')) == 81
         source.write_text('(let 1x 3 1x)')
         invoke(source,binary,ok=False)
+        for text in ['(+ x; ignore ) tokens\ny) ; end', '(+ x; comment\r\ny)', '(+ x y);end']:
+            source.write_text(text)
+            assert int(invoke('eval',source,'3','4','0')) == 7, text
         for text in ['0x','0x10000000000000000','0xfg','0xffextra','-0x1']:
             source.write_text(text)
             binary.write_bytes(b'preserve')