syntax: accept {,n} as an equivalent to {0,n}

Most regular expression engines don't accept the `{,n}` syntax, but
some other do it (namely Python's `re` library). This introduces a new
parser configuration option that enables the `{,n}` syntax.

PR #1086

Author: plusvic

regex-syntax/src/ast/parse.rs

@@ -124,6 +124,7 @@ pub struct ParserBuilder {
     ignore_whitespace: bool,
     nest_limit: u32,
     octal: bool,
+    empty_min_range: bool,
 }
 
 impl Default for ParserBuilder {
@@ -139,6 +140,7 @@ impl ParserBuilder {
             ignore_whitespace: false,
             nest_limit: 250,
             octal: false,
+            empty_min_range: false,
         }
     }
 
@@ -149,6 +151,7 @@ impl ParserBuilder {
             capture_index: Cell::new(0),
             nest_limit: self.nest_limit,
             octal: self.octal,
+            empty_min_range: self.empty_min_range,
             initial_ignore_whitespace: self.ignore_whitespace,
             ignore_whitespace: Cell::new(self.ignore_whitespace),
             comments: RefCell::new(vec![]),
@@ -221,6 +224,18 @@ impl ParserBuilder {
         self.ignore_whitespace = yes;
         self
     }
+
+    /// Allow using `{,n}` as an equivalent to `{0,n}`.
+    ///
+    /// When enabled, the parser accepts `{,n}` as valid syntax for `{0,n}`.
+    /// Most regular expression engines don't support the `{,n}` syntax, but
+    /// some others do it, namely Python's `re` library.
+    ///
+    /// This is disabled by default.
+    pub fn empty_min_range(&mut self, yes: bool) -> &mut ParserBuilder {
+        self.empty_min_range = yes;
+        self
+    }
 }
 
 /// A regular expression parser.
@@ -246,6 +261,9 @@ pub struct Parser {
     /// The initial setting for `ignore_whitespace` as provided by
     /// `ParserBuilder`. It is used when resetting the parser's state.
     initial_ignore_whitespace: bool,
+    /// Whether the parser supports `{,n}` repetitions as an equivalent to
+    /// `{0,n}.`
+    empty_min_range: bool,
     /// Whether whitespace should be ignored. When enabled, comments are
     /// also permitted.
     ignore_whitespace: Cell<bool>,
@@ -1114,32 +1132,48 @@ impl<'s, P: Borrow<Parser>> ParserI<'s, P> {
             self.parse_decimal(),
             ast::ErrorKind::DecimalEmpty,
             ast::ErrorKind::RepetitionCountDecimalEmpty,
-        )?;
-        let mut range = ast::RepetitionRange::Exactly(count_start);
+        );
         if self.is_eof() {
             return Err(self.error(
                 Span::new(start, self.pos()),
                 ast::ErrorKind::RepetitionCountUnclosed,
             ));
         }
-        if self.char() == ',' {
+        let range = if self.char() == ',' {
             if !self.bump_and_bump_space() {
                 return Err(self.error(
                     Span::new(start, self.pos()),
                     ast::ErrorKind::RepetitionCountUnclosed,
                 ));
             }
             if self.char() != '}' {
+                let count_start = match count_start {
+                    Ok(c) => c,
+                    Err(err)
+                        if err.kind
+                            == ast::ErrorKind::RepetitionCountDecimalEmpty =>
+                    {
+                        if self.parser().empty_min_range {
+                            0
+                        } else {
+                            return Err(err);
+                        }
+                    }
+                    err => err?,
+                };
                 let count_end = specialize_err(
                     self.parse_decimal(),
                     ast::ErrorKind::DecimalEmpty,
                     ast::ErrorKind::RepetitionCountDecimalEmpty,
                 )?;
-                range = ast::RepetitionRange::Bounded(count_start, count_end);
+                ast::RepetitionRange::Bounded(count_start, count_end)
             } else {
-                range = ast::RepetitionRange::AtLeast(count_start);
+                ast::RepetitionRange::AtLeast(count_start?)
             }
-        }
+        } else {
+            ast::RepetitionRange::Exactly(count_start?)
+        };
+
         if self.is_eof() || self.char() != '}' {
             return Err(self.error(
                 Span::new(start, self.pos()),
@@ -2459,6 +2493,11 @@ mod tests {
         ParserI::new(parser, pattern)
     }
 
+    fn parser_empty_min_range(pattern: &str) -> ParserI<'_, Parser> {
+        let parser = ParserBuilder::new().empty_min_range(true).build();
+        ParserI::new(parser, pattern)
+    }
+
     fn parser_nest_limit(
         pattern: &str,
         nest_limit: u32,
@@ -3376,6 +3415,20 @@ bar
                 ast: Box::new(lit('a', 0)),
             }))
         );
+        assert_eq!(
+            parser_empty_min_range(r"a{,9}").parse(),
+            Ok(Ast::repetition(ast::Repetition {
+                span: span(0..5),
+                op: ast::RepetitionOp {
+                    span: span(1..5),
+                    kind: ast::RepetitionKind::Range(
+                        ast::RepetitionRange::Bounded(0, 9)
+                    ),
+                },
+                greedy: true,
+                ast: Box::new(lit('a', 0)),
+            }))
+        );
         assert_eq!(
             parser_ignore_whitespace(r"a{5,9} ?").parse(),
             Ok(Ast::repetition(ast::Repetition {
@@ -4596,8 +4649,8 @@ bar
         assert_eq!(
             parser(r"\b{ ").parse().unwrap_err(),
             TestError {
-                span: span(4..4),
-                kind: ast::ErrorKind::RepetitionCountDecimalEmpty,
+                span: span(2..4),
+                kind: ast::ErrorKind::RepetitionCountUnclosed,
             }
         );
         // In this case, we got some valid chars that makes it look like the