Uppercasing Can Change String Length

Most code assumes case conversion is a one-to-one character mapping. It is not, and the assumption breaks in ways that are easy to miss.

One character becomes two

German ß uppercases to SS. Certain ligatures expand similarly. Any code allocating a fixed-size buffer based on the input length, or validating that a field still fits a column after uppercasing, can overflow on real input.

Not reversible

Uppercase then lowercase does not return the original string. Once ß has become SS, the information is gone. Never store only the uppercased form of data you will need to display.

Locale applies here too

The Turkish dotless i works in this direction as well: lowercase i uppercases to İ in Turkish and I elsewhere.

For comparison, prefer folding

If the goal is caseless matching, case folding is the operation designed for it. Uppercasing both sides of a comparison is a common shortcut that produces wrong answers on exactly the inputs nobody tests with.

Try it: Convert Text to Uppercase on SeoWolf's Notepad.