Handle the empty pre and post pattern

src/parser/match_rule.rs

@@ -32,6 +32,7 @@ pub enum Attribute {
 
 #[derive(Debug, PartialEq)]
 pub enum Pattern {
+    Empty,
     Characters(String),
     Boundary,
     Any,
@@ -218,9 +219,13 @@ impl<'a> PatternParser<'a> {
 
     pub fn pattern(&mut self) -> Result<Patterns, ParseError> {
         let mut patterns: Patterns = Vec::new();
-        while self.chars.peek().is_some() {
-            patterns.push(self.pattern_with_quantifier()?);
-        }
+	if self.chars.next_if(|&c| c == '-').is_some() {
+	    patterns.push(Pattern::Empty);
+	} else {
+            while self.chars.peek().is_some() {
+		patterns.push(self.pattern_with_quantifier()?);
+            }
+	}
         Ok(patterns)
     }
 }
@@ -333,5 +338,9 @@ mod tests {
             PatternParser::new("a**").pattern(),
             Err(ParseError::MissingPatternBeforeQuantifier)
         );
+        assert_eq!(
+            PatternParser::new("-").pattern(),
+	    Ok(vec![Pattern::Empty])
+        );
     }
 }