strings.bif/do_split_string: Pass bol and eol to MatchPrefix()

This allows better control of BOL and EOL. MatchPrefix() / LongestMatch()
always start with BOL.

Closes #3455

src/strings.bif

@@ -280,15 +280,23 @@ static zeek::VectorValPtr do_split_string(zeek::StringVal* str_val,
 	int num_sep = 0;
 
 	int offset = 0;
+	bool bol = true;
+	const bool eol = true;
+
 	while ( n >= 0 )
 		{
 		offset = 0;
 		// Find next match offset.
 		int end_of_match = 0;
-		while ( n > 0 &&
+		while ( n > 0 )
-		        (end_of_match = re->MatchPrefix(s + offset, n)) <= 0 )
 			{
-			// Move on to next byte.
+			end_of_match = re->MatchPrefix(s + offset, n, bol, eol);
+			if ( end_of_match > 0 )
+				break;
+
+			// Move on to next byte, use BOL only on the byte such that
+			// a BOL anchored pattern won't be matched anywhere else.
+			bol = false;
 			++offset;
 			--n;
 			}

testing/btest/Baseline/bifs.split_string/out

@@ -31,3 +31,9 @@ A
  C 
 =
  D
+test, ^est, [test]
+test, tes$, [test]
+test, ^test$, [, test, ]
+aa bb cc, / ?/, [aa, bb, cc]
+aa bb cc, / ?/, [aa,  , bb,  , cc]
+aa  bb   cc, / +/, [aa, bb, cc]

testing/btest/bifs/split_string.zeek

@@ -34,3 +34,46 @@ event zeek_init()
 	pat = /=/;
 	print_string_vector(split_string_all(a, pat));
 	}
+
+event zeek_init() &priority=-5
+	{
+	# Anchor testing.
+	local r = split_string_n("test", /^est/, T, 1);
+	assert |r| == 1;
+	assert r[0] == "test";
+	print "test", "^est", r;
+
+	r = split_string_n("test", /tes$/, T, 1);
+	assert |r| == 1;
+	assert r[0] == "test";
+	print "test", "tes$", r;
+
+	r = split_string_n("test", /^test$/, T, 1);
+	assert |r| == 3;
+	assert r[0] == "";
+	assert r[1] == "test";
+	assert r[2] == "";
+	print "test", "^test$", r;
+
+	r = split_string_n("aa bb cc", / ?/, F, 0);
+	assert |r| == 3;
+	assert r[0] == "aa";
+	assert r[1] == "bb";
+	assert r[2] == "cc";
+	print "aa bb cc", "/ ?/", r;
+
+	r = split_string_n("aa bb cc", / ?/, T, 0);
+	assert |r| == 5;
+	assert r[0] == "aa";
+	assert r[1] == " ";
+	assert r[2] == "bb";
+	assert r[3] == " ";
+	print "aa bb cc", "/ ?/", r;
+
+	r = split_string_n("aa  bb    cc", / +/, F, 0);
+	assert |r| == 3;
+	assert r[0] == "aa";
+	assert r[1] == "bb";
+	assert r[2] == "cc";
+	print "aa  bb   cc", "/ +/", r;
+	}