Caesar Cipher Encoder & Decoder

The Caesar cipher is a substitution cipher named after Julius Caesar, who reportedly used it for military messages around 50 BC. Each letter moves a fixed number of positions through the 26-letter English alphabet, wrapping around from Z back to A. This tool implements exactly that with a regular expression that matches only [a-zA-Z]. Uppercase and lowercase are handled separately so case is always preserved, and the math uses modular arithmetic — ((code + shift) mod 26) — that correctly wraps in both directions, so a negative shift simply decodes. Because the pattern only touches ASCII letters A through Z, digits, spaces, punctuation, emoji, and accented or non-Latin characters (such as é, ñ, or Cyrillic) pass straight through unchanged. There is no separate encrypt or decrypt button: encoding with shift n and decoding with shift -n are the same operation. Note that this is a teaching and puzzle tool, not real security. With only 25 distinct keys, a Caesar cipher is broken in seconds by trying every shift (brute force) or by frequency analysis, so never use it to protect anything sensitive.