Pattern Transformation (translate, pipelines, replace)

Pattern transformation maps whole strings through substitutions—character maps (str.translate), multi-step pipelines, or regex replacements. Think declaratively: what is the alphabet before and after?

Why this shows up in the real world

Romanization systems map Devanagari to Latin letters by table. PII redaction replaces digit runs with * while preserving length metadata.

Core idea (explained for students)

Use str.maketrans for one-to-one char maps. For whole-word swaps, re.sub with a callback keeps logic localized.

Try this in Python

def rot13(s: str) -> str:
    import codecs

    return codecs.encode(s, "rot13")


print(rot13("uryyb"))

Common mistakes

• Collisions when two source chars map to the same target unintentionally.
• Chained replaces where order matters (ab→x then ba→y).

Key takeaways

• Prefer immutable transforms (return new string) over in-place mutation lists unless performance requires buffers.
• Snapshot inputs before multi-pass transforms for debugging.