Add some phonenumberutil functions

src/string_util.rs

@@ -0,0 +1,36 @@
+use std::borrow::Cow;
+
+/// Strips prefix of given string Cow. Returns option with `Some` if 
+/// prefix found and stripped.
+/// 
+/// Calls `drain` if string is owned and returns slice if string is borrowed
+pub fn strip_cow_prefix<'a>(cow: Cow<'a, str>, prefix: &str) -> Option<Cow<'a, str>> {
+    match cow {
+        Cow::Borrowed(s) => s.strip_prefix(prefix).map(| s | Cow::Borrowed(s)),
+        Cow::Owned(mut s) => {
+            if s.starts_with(prefix) {
+                s.drain(0..prefix.len());                               
+                return Some(Cow::Owned(s));
+            }
+            None
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::borrow::Cow;
+
+    use crate::string_util::strip_cow_prefix;
+
+    #[test]
+    fn test_usage() {
+        let str_to_strip = Cow::Owned("test0:test".to_owned());
+        let stripped = strip_cow_prefix(str_to_strip, "test0");
+        assert_eq!(stripped, Some(Cow::Owned(":test".to_owned())));
+
+        let str_to_strip = Cow::Owned("test:test0".to_owned());
+        let stripped = strip_cow_prefix(str_to_strip, "test0");
+        assert_eq!(stripped, None)
+    }
+}