Make sure text markup doesn't have interior spaces.

src/parser/util.rs

@@ -151,6 +151,28 @@ pub fn start_of_line<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'
     Ok((input, ()))
 }
 
+/// Check that we are at the start of a line
+#[tracing::instrument(ret, level = "debug")]
+pub fn preceded_by_whitespace<'r, 's>(
+    context: Context<'r, 's>,
+    input: &'s str,
+) -> Res<&'s str, ()> {
+    let document_root = context.get_document_root().unwrap();
+    let preceding_character = get_one_before(document_root, input)
+        .map(|slice| slice.chars().next())
+        .flatten();
+    match preceding_character {
+        Some('\n') | Some('\r') | Some(' ') | Some('\t') => {}
+        // If None, we are at the start of the file which is not allowed
+        None | Some(_) => {
+            return Err(nom::Err::Error(CustomError::MyError(MyError(
+                "Not preceded by whitespace.",
+            ))));
+        }
+    };
+    Ok((input, ()))
+}
+
 /// Pull one non-whitespace character.
 ///
 /// This function only operates on spaces, tabs, carriage returns, and line feeds. It does not handle fancy unicode whitespace.