fix(formatting): split on last match

the implementation doesn't look very performant,
since it calls rfind and then rsplit_once.
using split_at to replace the second call is a little more difficult tho

src/formatting.rs

@@ -3,7 +3,7 @@ use rug::Float;
 use crate::nodes::{self, Error};
 
 pub fn pretty_eval(input: &str, max_digits: usize) -> Result<String, Error> {
-    let (prefix, deliminator, input) = input.rsplit_once_any(&["plus.minus ", "& ", "= "]);
+    let (prefix, deliminator, input) = input.rsplit_once_last(&["= ", "& ", "plus.minus "]);
 
     // no need to remove the character here,
     // as we ignore it in all other logic already
@@ -32,22 +32,25 @@ pub fn pretty_eval(input: &str, max_digits: usize) -> Result<String, Error> {
 }
 
 trait StrExt {
-    fn rsplit_once_any<'a>(
+    fn rsplit_once_last<'a>(
         &'a self,
         delimiters: &[&'static str],
     ) -> (&'a str, &'static str, &'a str);
 }
 impl StrExt for str {
-    fn rsplit_once_any<'a>(
+    fn rsplit_once_last<'a>(
         &'a self,
         delimiters: &[&'static str],
     ) -> (&'a str, &'static str, &'a str) {
-        for delimiter in delimiters {
-            if let Some((prefix, suffix)) = self.rsplit_once(delimiter) {
-                return (prefix, delimiter, suffix);
-            }
-        }
-        return ("", "", self);
+        delimiters
+            .iter()
+            .filter_map(|delimiter| self.rfind(delimiter).map(|i| (delimiter, i)))
+            .max_by_key(|(_, i)| *i)
+            .map(|(delimiter, _)| {
+                let (prefix, suffix) = self.rsplit_once(delimiter).unwrap();
+                (prefix, *delimiter, suffix)
+            })
+            .unwrap_or(("", "", self))
     }
 }
 
@@ -124,6 +127,14 @@ mod tests {
         assert_eq!(pretty_eval("-11", MAX_DIGITS).unwrap(), "-11");
     }
 
+    #[test]
+    fn complex_prefix() {
+        assert_eq!(
+            pretty_eval("plus.minus 5 & = 5", MAX_DIGITS).unwrap(),
+            "plus.minus 5 & = 5"
+        );
+    }
+
     #[test]
     fn basic_get_significant_digits() {
         let many_zeros = eval("14.00643 + 1.00784 * 4 + 35.446 + 15.9994 * 4");