Almost working implementation for brackets, stuck on last test

Cargo.toml

@@ -19,7 +19,7 @@ clap = "2.2.1"
 handlebars = "0.20.0"
 rustc-serialize = "0.3.18"
 pulldown-cmark = "0.0.8"
-nom = "1.2.3"
+nom = "1.2.4"
 
 # Watch feature
 notify = { version = "2.5.5", optional = true }

src/parse/nom.rs

@@ -1,4 +1,4 @@
-use nom::{self, IResult, alphanumeric};
+use nom::{self, IResult};
 
 use std::str;
 use std::str::FromStr;
@@ -15,6 +15,7 @@ fn link(i: &[u8]) -> IResult<&[u8], Link> {
 }
 
 /// Parser for parsing the title part of the link: [title](destination)
+///                                                ^^^^^^^
 ///
 /// From the Common Mark spec (http://spec.commonmark.org/0.26/#links):
 ///
@@ -22,9 +23,42 @@ fn link(i: &[u8]) -> IResult<&[u8], Link> {
 ///   (a) they are backslash-escaped or
 ///   (b) they appear as a matched pair of brackets,
 ///       with an open bracket [, a sequence of zero or more inlines, and a close bracket ].
-///                                        ^^^^^^^
+///
 fn link_text(i: &[u8]) -> IResult<&[u8], String> {
-    map_res!(i, map_res!(delimited!(char!('['), is_not!("[]"), char!(']')), str::from_utf8), FromStr::from_str)
+    map_res!(i,
+        map_res!(
+            delimited!(
+                // Begin with '['
+                char!('['),
+                // Followed by anything that is not '[' or ']'
+                // Make sure to allow escaped brackets and balanced brackets
+                recognize!(many1!(alt!(not_unescaped_bracket | balanced_brackets))),
+                // End with ']'
+                char!(']')
+            ),
+            str::from_utf8
+        ),
+        FromStr::from_str
+    )
+}
+
+#[cfg_attr(rustfmt, rustfmt_skip)]
+fn balanced_brackets(i: &[u8]) -> IResult<&[u8], &[u8]> {
+    recognize!(i,
+        delimited!(
+            // Begin with '['
+            char!('['),
+            // Followed by anything that is not '[' or ']'
+            // Make sure to allow escaped brackets and balanced brackets
+            many0!(alt!(not_unescaped_bracket | balanced_brackets)),
+            // End with ']'
+            char!(']')
+        )
+    )
+}
+
+fn not_unescaped_bracket(i: &[u8]) -> IResult<&[u8], &[u8]> {
+    escaped!(i, is_not!("\\[]"), '\\', is_a_bytes!(&b"[]"[..]))
 }
 
 /// Parser for parsing the destination part of the link: [title](destination)
@@ -37,6 +71,33 @@ fn link_destination(i: &[u8]) -> IResult<&[u8], String> {
 mod tests {
 
     use nom::{self, IResult};
+    use nom::Err::{NodePosition, Position};
+    use nom::ErrorKind::Escaped;
+
+    // Tests not_unescaped_bracket
+    #[test]
+    fn not_unescaped_bracket() {
+        assert_eq!(super::not_unescaped_bracket(b"a"), IResult::Done(&b""[..], &b"a"[..]));
+        assert_eq!(super::not_unescaped_bracket(b"-"), IResult::Done(&b""[..], &b"-"[..]));
+        assert_eq!(super::not_unescaped_bracket(b"\\["), IResult::Done(&b""[..], &b"\\["[..]));
+        assert_eq!(super::not_unescaped_bracket(b"]"), IResult::Error(NodePosition(Escaped, &[93][..], Box::new(Position(Escaped, &[93][..])))));
+    }
+
+    // Tests for balanced brackets
+    #[test]
+    fn balanced_brackets() {
+        assert_eq!(super::balanced_brackets(b"[a]"), nom::IResult::Done(&b""[..], &b"[a]"[..]));
+    }
+
+    #[test]
+    fn balanced_brackets_empty() {
+        assert_eq!(super::balanced_brackets(b"[]"), nom::IResult::Done(&b""[..], &b"[]"[..]));
+    }
+
+    #[test]
+    fn balanced_brackets_nested() {
+        assert_eq!(super::balanced_brackets(b"[abc[a]]"), nom::IResult::Done(&b""[..], &b"[abc[a]]"[..]));
+    }
 
     // Tests for link_text
 
@@ -66,6 +127,6 @@ mod tests {
     #[test]
     fn link_text_brackets() {
         assert_eq!(super::link_text(b"[Intro[]]"), nom::IResult::Done(&b""[..], String::from("Intro[]")));
-        assert_eq!(super::link_text(br"[Intro\[]"), nom::IResult::Done(&b""[..], String::from("Intro[")));
+        assert_eq!(super::link_text(br"[Intro\]]"), nom::IResult::Done(&b""[..], String::from("Intro[")));
     }
 }