diff --git a/NEWS b/NEWS
index bd056bc781..99fd3b494a 100644
--- a/NEWS
+++ b/NEWS
@@ -24,6 +24,10 @@ Breaking Changes
 New Functionality
 -----------------
 
+- Added support for the /s regular expression modifier. Using this modifier in
+  patterns in Zeek scripts will cause the '.' character to also match newline
+  characters.
+
 Changed Functionality
 ---------------------
 
diff --git a/src/RE.cc b/src/RE.cc
index 13d21e59e5..e6389de943 100644
--- a/src/RE.cc
+++ b/src/RE.cc
@@ -18,6 +18,7 @@ zeek::detail::CCL* zeek::detail::curr_ccl = nullptr;
 zeek::detail::Specific_RE_Matcher* zeek::detail::rem = nullptr;
 zeek::detail::NFA_Machine* zeek::detail::nfa = nullptr;
 bool zeek::detail::case_insensitive = false;
+bool zeek::detail::re_single_line = false;
 
 extern int RE_parse(void);
 extern void RE_set_input(const char* str);
@@ -32,6 +33,7 @@ Specific_RE_Matcher::Specific_RE_Matcher(match_type arg_mt, bool arg_multiline)
 	: mt(arg_mt), multiline(arg_multiline), equiv_class(NUM_SYM)
 	{
 	any_ccl = nullptr;
+	single_line_ccl = nullptr;
 	dfa = nullptr;
 	ecs = nullptr;
 	accepted = new AcceptingSet();
@@ -46,10 +48,22 @@ Specific_RE_Matcher::~Specific_RE_Matcher()
 	delete accepted;
 	}
 
-CCL* Specific_RE_Matcher::AnyCCL()
+CCL* Specific_RE_Matcher::AnyCCL(bool single_line_mode)
 	{
+	if ( single_line_mode )
+		{
+		if ( ! single_line_ccl )
+			{
+			single_line_ccl = new CCL();
+			single_line_ccl->Negate();
+			EC()->CCL_Use(single_line_ccl);
+			}
+
+		return single_line_ccl;
+		}
+
 	if ( ! any_ccl )
-		{ // Create the '.' character class.
+		{
 		any_ccl = new CCL();
 		if ( ! multiline )
 			any_ccl->Add('\n');
@@ -98,6 +112,12 @@ void Specific_RE_Matcher::MakeCaseInsensitive()
 	pattern_text = util::fmt(fmt, pattern_text.c_str());
 	}
 
+void Specific_RE_Matcher::MakeSingleLine()
+	{
+	const char fmt[] = "(?s:%s)";
+	pattern_text = util::fmt(fmt, pattern_text.c_str());
+	}
+
 bool Specific_RE_Matcher::Compile(bool lazy)
 	{
 	if ( pattern_text.empty() )
@@ -394,13 +414,10 @@ static RE_Matcher* matcher_merge(const RE_Matcher* re1, const RE_Matcher* re2, c
 	const char* text1 = re1->PatternText();
 	const char* text2 = re2->PatternText();
 
-	int n = strlen(text1) + strlen(text2) + strlen(merge_op) + 32 /* slop */;
+	size_t n = strlen(text1) + strlen(text2) + strlen(merge_op) + 32 /* slop */;
 
-	char* merge_text = new char[n];
-	snprintf(merge_text, n, "(%s)%s(%s)", text1, merge_op, text2);
-
-	RE_Matcher* merge = new RE_Matcher(merge_text);
-	delete[] merge_text;
+	std::string merge_text = util::fmt("(%s)%s(%s)", text1, merge_op, text2);
+	RE_Matcher* merge = new RE_Matcher(merge_text.c_str());
 
 	merge->Compile();
 
@@ -461,6 +478,14 @@ void RE_Matcher::MakeCaseInsensitive()
 	is_case_insensitive = true;
 	}
 
+void RE_Matcher::MakeSingleLine()
+	{
+	re_anywhere->MakeSingleLine();
+	re_exact->MakeSingleLine();
+
+	is_single_line = true;
+	}
+
 bool RE_Matcher::Compile(bool lazy)
 	{
 	return re_anywhere->Compile(lazy) && re_exact->Compile(lazy);
@@ -523,15 +548,51 @@ TEST_SUITE("re_matcher")
 		CHECK(match.MatchExactly("aBc"));
 		CHECK(match.MatchExactly("nop"));
 		CHECK_FALSE(match.MatchExactly("NoP"));
+		}
 
-		// TODO: this part isn't working at all. There's something about the second call
-		// to Compile() that's breaking something.
-		// match.MakeCaseInsensitive();
-		// match.Compile();
-		// CHECK(strcmp(match.PatternText(), "(?i:((?i:^?([a-m]+)$?))|(^?([n-z]+)$?))") == 0);
-		// CHECK(match.MatchExactly("aBc"));
-		// CHECK(match.MatchExactly("nop"));
-		// CHECK(match.MatchExactly("NoP"));
+	TEST_CASE("single_line_mode")
+		{
+		RE_Matcher match(".*");
+		match.MakeSingleLine();
+		match.Compile();
+
+		CHECK(strcmp(match.PatternText(), "(?s:^?(.*)$?)") == 0);
+		CHECK(match.MatchExactly("abc\ndef"));
+
+		RE_Matcher match2("fOO.*bAR");
+		match2.MakeSingleLine();
+		match2.Compile();
+
+		CHECK(strcmp(match2.PatternText(), "(?s:^?(fOO.*bAR)$?)") == 0);
+		CHECK(match.MatchExactly("fOOab\ncdbAR"));
+
+		RE_Matcher match3("b.r");
+		match3.MakeSingleLine();
+		match3.Compile();
+		CHECK(match3.MatchExactly("bar"));
+		CHECK(match3.MatchExactly("b\nr"));
+
+		RE_Matcher match4("a.c");
+		match4.MakeSingleLine();
+		match4.AddPat("def");
+		match4.Compile();
+		CHECK(match4.MatchExactly("abc"));
+		CHECK(match4.MatchExactly("a\nc"));
+		}
+
+	TEST_CASE("disjunction")
+		{
+		RE_Matcher match1("a.c");
+		match1.MakeSingleLine();
+		match1.Compile();
+		RE_Matcher match2("def");
+		match2.Compile();
+		auto dj = detail::RE_Matcher_disjunction(&match1, &match2);
+		CHECK(dj->MatchExactly("abc"));
+		CHECK(dj->MatchExactly("a.c"));
+		CHECK(dj->MatchExactly("a\nc"));
+		CHECK(dj->MatchExactly("def"));
+		delete dj;
 		}
 	}
 
diff --git a/src/RE.h b/src/RE.h
index c71b8dd746..28c343f655 100644
--- a/src/RE.h
+++ b/src/RE.h
@@ -33,6 +33,7 @@ class Specific_RE_Matcher;
 class CCL;
 
 extern bool case_insensitive;
+extern bool re_single_line;
 extern CCL* curr_ccl;
 extern NFA_Machine* nfa;
 extern Specific_RE_Matcher* rem;
@@ -65,7 +66,7 @@ public:
 	void AddPat(const char* pat);
 
 	void MakeCaseInsensitive();
-	void SetSingleLineMode();
+	void MakeSingleLine();
 
 	void SetPat(const char* pat) { pattern_text = pat; }
 
@@ -91,7 +92,7 @@ public:
 		return nullptr;
 		}
 	CCL* LookupCCL(int index) { return ccl_list[index]; }
-	CCL* AnyCCL();
+	CCL* AnyCCL(bool single_line_mode = false);
 
 	void ConvertCCLs();
 
@@ -147,8 +148,10 @@ protected:
 	EquivClass equiv_class;
 	int* ecs;
 	DFA_Machine* dfa;
-	CCL* any_ccl;
 	AcceptingSet* accepted;
+
+	CCL* any_ccl;
+	CCL* single_line_ccl;
 	};
 
 class RE_Match_State
@@ -208,6 +211,9 @@ public:
 	void MakeCaseInsensitive();
 	bool IsCaseInsensitive() const { return is_case_insensitive; }
 
+	void MakeSingleLine();
+	bool IsSingleLine() const { return is_single_line; }
+
 	bool Compile(bool lazy = false);
 
 	// Returns true if s exactly matches the pattern, false otherwise.
@@ -243,6 +249,7 @@ protected:
 	detail::Specific_RE_Matcher* re_exact;
 
 	bool is_case_insensitive = false;
+	bool is_single_line = false;
 	};
 
 	} // namespace zeek
diff --git a/src/parse.y b/src/parse.y
index 637e40c318..1336d177cf 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -922,8 +922,7 @@ expr:
 				re->MakeCaseInsensitive();
 
 			if ( $4.single_line )
-				{
-				}
+				re->MakeSingleLine();
 
 			re->Compile();
 			$$ = new ConstExpr(make_intrusive<PatternVal>(re));
diff --git a/src/re-parse.y b/src/re-parse.y
index 5d8d2e9a63..7a38820f90 100644
--- a/src/re-parse.y
+++ b/src/re-parse.y
@@ -20,7 +20,7 @@ namespace zeek::detail {
 void yyerror(const char msg[]);
 %}
 
-%token TOK_CHAR TOK_NUMBER TOK_CCL TOK_CCE TOK_CASE_INSENSITIVE
+%token TOK_CHAR TOK_NUMBER TOK_CCL TOK_CCE TOK_CASE_INSENSITIVE TOK_SINGLE_LINE
 
 %union {
 	int int_val;
@@ -112,7 +112,8 @@ singleton	:  singleton '*'
 
 		|  '.'
 			{
-			$$ = new zeek::detail::NFA_Machine(new zeek::detail::NFA_State(zeek::detail::rem->AnyCCL()));
+			$$ = new zeek::detail::NFA_Machine(new zeek::detail::NFA_State(
+                zeek::detail::rem->AnyCCL(zeek::detail::re_single_line)));
 			}
 
 		|  full_ccl
@@ -134,6 +135,9 @@ singleton	:  singleton '*'
 		|  TOK_CASE_INSENSITIVE re ')'
 			{ $$ = $2; zeek::detail::case_insensitive = false; }
 
+		|  TOK_SINGLE_LINE re ')'
+			{ $$ = $2; zeek::detail::re_single_line = false; }
+
 		|  TOK_CHAR
 			{
 			auto sym = $1;
diff --git a/src/re-scan.l b/src/re-scan.l
index 494dc5b486..5bffd812a7 100644
--- a/src/re-scan.l
+++ b/src/re-scan.l
@@ -116,6 +116,7 @@ CCL_EXPR	("[:"[[:alpha:]]+":]")
 			}
 
 	"(?i:"		zeek::detail::case_insensitive = true; return TOK_CASE_INSENSITIVE;
+	"(?s:"		zeek::detail::re_single_line = true; return TOK_SINGLE_LINE;
 
 	[a-zA-Z]	{
 			if ( zeek::detail::case_insensitive )
diff --git a/src/scan.l b/src/scan.l
index 508c21cbfe..169af2932b 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -570,7 +570,7 @@ F	RET_CONST(zeek::val_mgr->False()->Ref())
 <RE>(\/[is]{0,2})	{
 	BEGIN(INITIAL);
 
-	if (strlen(yytext) == 2)
+	if ( strlen(yytext) == 2 )
 		{
 		yylval.re_modes.ignore_case = (yytext[1] == 'i');
 		yylval.re_modes.single_line = (yytext[1] == 's');
diff --git a/testing/btest/Baseline/language.pattern/out b/testing/btest/Baseline/language.pattern/out
index 8bbf981c12..5c1441502b 100644
--- a/testing/btest/Baseline/language.pattern/out
+++ b/testing/btest/Baseline/language.pattern/out
@@ -33,3 +33,7 @@ case-sensitive pattern (PASS)
 (?i:...) pattern construct (PASS)
 (?i:...) pattern construct (FAIL)
 (?i:...) pattern construct (PASS)
+/s missing (PASS)
+/s pattern modifier (PASS)
+/s pattern disjunction (PASS)
+/s pattern concatenation (PASS)
diff --git a/testing/btest/language/pattern.zeek b/testing/btest/language/pattern.zeek
index 05a84e713c..2567afc884 100644
--- a/testing/btest/language/pattern.zeek
+++ b/testing/btest/language/pattern.zeek
@@ -65,4 +65,9 @@ event zeek_init()
 	test_case( "(?i:...) pattern construct", /foo|(?i:bar)/ in "xFOoy" );
 	test_case( "(?i:...) pattern construct", /foo|(?i:bar)/ | /foo/i in "xFOoy" );
 
+	test_case( "/s missing", /fOO.*bAR/ != "fOOab\ncdbAR");
+	test_case( "/s pattern modifier", /fOO.*bAR/s == "fOOab\ncdbAR");
+	test_case( "/s pattern disjunction", /b.r/s | /bez/ == "b\nr" );
+	test_case( "/s pattern concatenation", /b.r/s & /bez/ == "b\nrbez" );
+
 }