isLetter

isLetter is a predicate used in programming to determine whether a given character is a letter rather than a numeral, punctuation, or symbol. In most contexts, isLetter is tied to Unicode-aware definitions of letters, typically including the Unicode categories Lu (uppercase), Ll (lowercase), Lt (titlecase), Lm (modifier letter), and Lo (other letter), with some implementations also counting certain Letter_Number (Nl) code points as letters. Different languages expose similar functionality under different names, and the exact scope can vary by library.

In common language APIs, isLetter is implemented as follows:

- Java: Character.isLetter(int codePoint) returns true if the code point is a Unicode letter.

- C#: Char.IsLetter(char c) returns true for Unicode letters.

- C/C++: isalpha(int ch) from ctype.h returns a nonzero value for alphabetic characters but is locale-dependent and not guaranteed to be Unicode-aware without additional libraries.

- JavaScript: There is no universal built-in isLetter; developers often use Unicode property escapes with regular expressions (for example, \p{L} with the u flag) to test for letters.

- Python: str.isalpha() checks whether all characters in a string are alphabetic; for a single-character string, it behaves similarly to an isLetter check.

Examples: 'A' and 'é' are letters, so isLetter would be true; '3' and '+' are not letters, so isLetter would be false. Edge cases include characters from extended scripts, combining marks, and characters outside the BMP; in UTF-16-based environments, handling may require proper code point processing rather than single code units.

See also: isAlpha, isDigit, Unicode general categories, character classification.