read-until-match: fix regression.

Commit 9aa751c8a4f845ef2d2bba091c81ffeded941afd
broke things.

This fix affects the function read-until-match,
scan-until-match and count-until-match which share
implementation.

* regex.c (scan_until_common): In the REGM_MATCH_DONE
and REGM_MATCH cases, we must push the character onto
the local stack, before doing the match = stack
assignment. Otherwise, it's possible that the stack
is empty and so no match is recorded. The REGM_FAIL
case will then behave as if no match was found, consuming
a character and continuing.

* txr.1: Codify an existing behavior: only non-empty matches
for the regex are considered by read-until-match.

* tests/015/regex.tl: New file. I am amazed to discover
that we don't seem to have a test suite for regexes at all.
Putting the tests here which confirm this fix and provide
coverage for some edge cases in read-until-match.

3 files changed, 21 insertions, 2 deletions

diff --git a/regex.c b/regex.c
index aaddec64..9437881e 100644
--- a/regex.c
+++ b/regex.c
@@ -3234,9 +3234,11 @@ static val scan_until_common(val self, val regex, val stream_in,
       regex_machine_reset(&regm);
       continue;
     case REGM_MATCH_DONE:
+      push(ch, &stack);
       match = stack;
       goto out_match;
     case REGM_MATCH:
+      push(ch, &stack);
       match = stack;
       continue;
     case REGM_INCOMPLETE:
diff --git a/tests/015/regex.tl b/tests/015/regex.tl
new file mode 100644
index 00000000..68058eea
--- /dev/null
+++ b/tests/015/regex.tl
@@ -0,0 +1,18 @@
+(load "../common")
+
+(defun rum (str regex : include-match) 
+  (with-in-string-stream (s str)
+    (list (read-until-match regex s include-match)
+          (read-until-match regex s include-match))))
+
+(mtest
+  (rum "a-b" #/-/) ("a" "b")
+  (rum "a-b" #/-/ t) ("a-" "b")
+  (rum "a----b" #/-+/) ("a" "b")
+  (rum "a----b" #/-+/ t) ("a----" "b")
+  (rum "a----b" #/-*/) ("a" "b")
+  (rum "a----b" #/-*/ t) ("a----" "b")
+  (rum "abc" #/-/) ("abc" nil)
+  (rum "abc" #/-/ t) ("abc" nil)
+  (rum "a___b___#c" #/_+#/) ("a___b" "c")
+  (rum "a___b___#c" #/_+#/ t) ("a___b___#" "c"))
diff --git a/txr.1 b/txr.1
index 277673c7..128fcebf 100644
--- a/txr.1
+++ b/txr.1
@@ -56676,10 +56676,9 @@ matched by
 is included in the returned string. It defaults to
 .codn nil .
 
-The accumulation of characters is terminated by a match on
+The accumulation of characters is terminated by a non-empty match on
 .metn regex ,
 the end of the stream, or an error.
-
 This means that characters are read from the stream and accumulated while the
 stream has more characters available, and while its prefix does not match
 .metn regex .