Changed behavior of SOS and EOS assertions depending on whether multiline mode is enabled or not

nfa.go

@@ -108,11 +108,15 @@ func (s State) checkAssertion(str []rune, idx int) bool {
 		return true
 	}
 	if s.assert == SOS {
-		return idx == 0
+		// Single-line mode: Beginning of string
+		// Multi-line mode: Previous character was newline
+		return idx == 0 || (multilineMode && (idx > 0 && str[idx-1] == '\n'))
 	}
 	if s.assert == EOS {
+		// Single-line mode: End of string
+		// Multi-line mode: current character is newline
 		// Index is at the end of the string, or it points to the last character which is a newline
-		return idx == len(str) || (idx == len(str)-1 && str[len(str)-1] == '\n')
+		return idx == len(str) || (multilineMode && str[idx] == '\n')
 	}
 	if s.assert == WBOUND {
 		return isWordBoundary(str, idx)