Define pre and post matchers for plain link.

src/parser/plain_link.rs

@@ -1,10 +1,45 @@
+use nom::branch::alt;
+use nom::character::complete::none_of;
+use nom::combinator::eof;
+use nom::combinator::recognize;
+
 use super::Context;
+use crate::error::CustomError;
+use crate::error::MyError;
 use crate::error::Res;
 use crate::parser::object::PlainLink;
+use crate::parser::util::get_one_before;
 use crate::parser::util::not_yet_implemented;
+use crate::parser::util::WORD_CONSTITUENT_CHARACTERS;
 
 #[tracing::instrument(ret, level = "debug")]
 pub fn plain_link<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, PlainLink<'s>> {
     not_yet_implemented()?;
     todo!();
 }
+
+#[tracing::instrument(ret, level = "debug")]
+pub fn pre<'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 {
+        // If None, we are at the start of the file which is fine
+        None => {}
+        Some(x) if !WORD_CONSTITUENT_CHARACTERS.contains(x) => {}
+        Some(_) => {
+            // Not at start of line, cannot be a heading
+            return Err(nom::Err::Error(CustomError::MyError(MyError(
+                "Not a valid pre character for plain link.",
+            ))));
+        }
+    };
+    Ok((input, ()))
+}
+
+#[tracing::instrument(ret, level = "debug")]
+pub fn post<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, ()> {
+    let (remaining, _) = alt((eof, recognize(none_of(WORD_CONSTITUENT_CHARACTERS))))(input)?;
+    Ok((remaining, ()))
+}