diff --git a/CHANGES b/CHANGES
index ce9f1852b2..1e1f0969a4 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,4 +1,11 @@
 
+3.1.0-dev.521 | 2020-01-31 14:18:17 -0800
+
+  * util: optimize expand_escape() by avoiding sscanf() (Max Kellermann)
+
+    sscanf() is notoriously slow, and the default scripts have lots of hex
+    escapes.  This patch can reduce Zeek's startup time by 3-9%.
+
 3.1.0-dev.519 | 2020-01-31 13:19:09 -0800
 
   * util: optimize tokenize_string() and normalize_path() (Max Kellermann)
diff --git a/VERSION b/VERSION
index 19a258a590..433ae06f33 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.1.0-dev.519
+3.1.0-dev.521
diff --git a/src/util.cc b/src/util.cc
index 28f95a002e..51bfc8639c 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -293,6 +293,26 @@ int streq(const char* s1, const char* s2)
 	return ! strcmp(s1, s2);
 	}
 
+static constexpr int parse_octal_digit(char ch) noexcept
+	{
+	if ( ch >= '0' && ch <= '7' )
+		return ch - '0';
+	else
+		return -1;
+	}
+
+static constexpr int parse_hex_digit(char ch) noexcept
+	{
+	if ( ch >= '0' && ch <= '9' )
+		return ch - '0';
+	else if ( ch >= 'a' && ch <= 'f' )
+		return 10 + ch - 'a';
+	else if ( ch >= 'A' && ch <= 'F' )
+		return 10 + ch - 'A';
+	else
+		return -1;
+	}
+
 int expand_escape(const char*& s)
 	{
 	switch ( *(s++) ) {
@@ -310,23 +330,32 @@ int expand_escape(const char*& s)
 		--s;	// put back the first octal digit
 		const char* start = s;
 
-		// Don't increment inside loop control
-		// because if isdigit() is a macro it might
-		// expand into multiple increments ...
+		// require at least one octal digit and parse at most three
 
-		// Here we define a maximum length for escape sequence
-		// to allow easy handling of string like: "^H0" as
-		// "\0100".
+		int result = parse_octal_digit(*s++);
 
-		for ( int len = 0; len < 3 && isascii(*s) && isdigit(*s);
-		      ++s, ++len)
-			;
-
-		int result;
-		if ( sscanf(start, "%3o", &result) != 1 )
+		if ( result < 0 )
 			{
-			reporter->Warning("bad octal escape: %s ", start);
-			result = 0;
+			reporter->Error("bad octal escape: %s", start);
+			return 0;
+			}
+
+		// second digit?
+		int digit = parse_octal_digit(*s);
+
+		if ( digit >= 0 )
+			{
+			result = (result << 3) | digit;
+			++s;
+
+			// third digit?
+			digit = parse_octal_digit(*s);
+
+			if ( digit >= 0 )
+				{
+				result = (result << 3) | digit;
+				++s;
+				}
 			}
 
 		return result;
@@ -337,15 +366,22 @@ int expand_escape(const char*& s)
 		const char* start = s;
 
 		// Look at most 2 characters, so that "\x0ddir" -> "^Mdir".
-		for ( int len = 0; len < 2 && isascii(*s) && isxdigit(*s);
-		      ++s, ++len)
-			;
 
-		int result;
-		if ( sscanf(start, "%2x", &result) != 1 )
+		int result = parse_hex_digit(*s++);
+
+		if ( result < 0 )
 			{
-			reporter->Warning("bad hexadecimal escape: %s", start);
-			result = 0;
+			reporter->Error("bad hexadecimal escape: %s", start);
+			return 0;
+			}
+
+		// second digit?
+		int digit = parse_hex_digit(*s);
+
+		if ( digit >= 0 )
+			{
+			result = (result << 4) | digit;
+			++s;
 			}
 
 		return result;