Using regex but will try to solve the problem without it.,

Cargo.toml

@@ -13,3 +13,4 @@ rpath = true
 [dependencies]
 nom = "7.1.1"
 itertools = "0.10.5"
+fancy-regex = { version = "0.10.0", features = ["perf-literal", "perf-dfa", "perf-inline", "perf-cache"] }

src/main.rs

@@ -44,8 +44,8 @@ fn main() {
         println!("Solution for problem 5 part 2: {max_part1}");
     }
     {
-        let file_path = "prob6_part1.txt";
-        let solution = prob6::solve_part1(file_path);
+        let _file_path = "prob6_part1.txt";
+        let solution = prob6::solve_part1(include_str!("../prob6_part1.txt"));
         println!("Solution for problem 6 part 1: {solution}");
     }
 }

src/prob6.rs

@@ -1,5 +1,45 @@
-pub fn solve_part1(file_path: &str) -> i32 {
-    let _content = std::fs::read_to_string(file_path).unwrap_or_else(|err| panic!("{err}"));
-    println!("{_content}");
-    0
+use std::collections::HashSet;
+
+use fancy_regex::Regex;
+
+/// Tries to find the 4 character match.
+///
+/// # Arguments
+///
+/// * `datagram`:
+///
+/// returns: ()
+///
+/// # Examples
+///
+/// ```
+///
+/// ```
+fn begin_datagram(datagram: &str) {
+    let mut letters = HashSet::<char>::new();
+    let mut primary_iter = datagram.chars().fuse();
+    let mut motif_iter = primary_iter.clone();
+    while letters.len() < 4 {
+        if let Some(tmp) = motif_iter.next() {
+            if !letters.insert(tmp) {
+                letters.clear();
+            }
+        } else {
+            break;
+        }
+    }
+}
+
+pub fn solve_part1(content: &str) -> i32 {
+    let _content = content;
+    return match Regex::new(r"^.*(.).*\1.*$") {
+        Ok(_reg) => {
+            println!("Regex is built.");
+            0
+        }
+        Err(err) => {
+            eprintln!("{err}");
+            0
+        }
+    };
 }