strings.bif/sub,gsub: Respect anchors in pattern

Anchors within pattern passed to sub() or gsub() were previously ignored,
replacing any occurrence of '<text>' even when '^<text>' was used as a
pattern.

This is a pretty user-visible change (and we even have anchored patterns
within the base scripts), but seems "the right thing to do".

Relates to #3455

NEWS

@@ -23,6 +23,10 @@ Changed Functionality
   end-of-line $ anchors. Previously, an anchored pattern would be matched anywhere
   in the input string.
 
+- The ``sub()`` and ``gsub()` functions now respect the beginning-of-line ^ and
+  end-of-line $ anchors. Previously, an anchored pattern would be matched anywhere
+  in the input string.
+
 Removed Functionality
 ---------------------
 

src/Val.cc

@@ -789,15 +789,22 @@ StringValPtr StringVal::Replace(RE_Matcher* re, const String& repl, bool do_all)
     vector<std::pair<int, int>> cut_points;
 
     int size = 0; // size of result
+    bool bol = true;
+    const bool eol = true;
 
     while ( n > 0 ) {
         // Find next match offset.
         int end_of_match;
-        while ( n > 0 && (end_of_match = re->MatchPrefix(&s[offset], n)) <= 0 ) {
+        while ( n > 0 ) {
+            end_of_match = re->MatchPrefix(&s[offset], n, bol, eol);
+            if ( end_of_match > 0 )
+                break;
+
             // This character is going to be copied to the result.
             ++size;
 
             // Move on to next character.
+            bol = false;
             ++offset;
             --n;
         }

testing/btest/Baseline/bifs.sub/.stderr

@@ -0,0 +1 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.

testing/btest/Baseline/bifs.sub/out

@@ -1,3 +1,14 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 that is a test
 that at a test
+test
+test
+foo
+foo
+tea
+tea test
+test tea
+test tea
+tea tea
+tea tea
+tea

testing/btest/bifs/sub.zeek

@@ -1,6 +1,8 @@
+# @TEST-DOC: Test the sub() and gsub() functions.
 #
 # @TEST-EXEC: zeek -b %INPUT >out
 # @TEST-EXEC: btest-diff out
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderr
 
 event zeek_init()
 	{
@@ -10,3 +12,53 @@ event zeek_init()
 	print sub(a, pat, "at");
 	print gsub(a, pat, "at");
 	}
+
+event zeek_init() &priority=-1
+	{
+	local r = sub("test", /^est/, "ea");
+	assert r == "test", r;
+	print r;
+
+	r = sub("test", /tes$/, "foo");
+	assert r == "test", r;
+	print r;
+
+	r = sub("test", /test/, "foo");
+	assert r == "foo", r;
+	print r;
+
+	r = sub("test", /^test$/, "foo");
+	assert r == "foo", r;
+	print r;
+
+	r = sub("test", /est$/, "ea");
+	assert r == "tea", r;
+	print r;
+	}
+
+event zeek_init() &priority=-2
+	{
+	local r = gsub("test test", /^test/, "tea");
+	assert r == "tea test", r;
+	print r;
+
+	r = gsub("test test", /test$/, "tea");
+	assert r == "test tea", r;
+	print r;
+
+	r = gsub("test test", /test$/, "tea");
+	assert r == "test tea", r;
+	print r;
+
+	r = gsub("test test", /test/, "tea");
+	assert r == "tea tea", r;
+	print r;
+
+	r = gsub("test test", /est/, "ea");
+	assert r == "tea tea", r;
+	print r;
+
+	r = gsub("test test", /^test test$/, "tea");
+	assert r == "tea", r;
+	print r;
+	}