Strings & Text

Why String and &str exist as separate types, the UTF-8 invariant Rust enforces on every str, and why byte-indexing a string is a trap.

`String` vs `&str` and the ownership split.

Why byte-indexing panics and how to slice safely.

Building, concatenation, and capacity.

`parse`, `FromStr`, and `to_string`.

`format!`, format specifiers, and custom `Display`.

Iterating text correctly.

The `regex` crate and pattern matching.

Efficient, correct, allocation-aware text handling.