C character classification is an operation provided by a group of functions in the ANSI C Standard Library for the C programming language. These functions are used to test characters for membership in a particular class of characters, such as alphabetic characters, control characters, etc. Both single-byte, and wide characters are supported.[1]
Early C-language programmers working on the Unix operating system developed programming idioms for classifying characters into different types. For example, for the ASCII character set, the following expression identifies a letter, when its value is true:
As this may be expressed in multiple formulations, it became desirable to introduce short, standardized forms of such tests that were placed in the system-wide header file ctype.h.
Unlike the above example, the character classification routines are not written as comparison tests. In most C libraries, they are written as static table lookups instead of macros or functions.
For example, an array of 256 eight-bit integers, arranged as bitfields, is created, where each bit corresponds to a particular property of the character, e.g., isdigit, isalpha. If the lowest-order bit of the integers corresponds to the isdigit property, the code could be written as
Early versions of Linux used a potentially faulty method similar to the first code sample:
This can cause problems if when the macro expands, the expression substituted for x has a side effect. For example, if one calls isdigit(x++) or isdigit(run_some_program). It is not immediately evident that the argument to isdigit is evaluated twice. For this reason, the table-based approach is generally used.
The functions that operate on single-byte characters are defined in ctype.h header file (cctype in C++). The functions that operate on wide characters are defined in wctype.h header file (cwctype in C++).
The classification is evaluated according to the effective locale.
| Byte character | Wide character | Description | |
|---|---|---|---|
[http://en.cppreference.com/w/c/string/byte/isalnum isalnum] | [http://en.cppreference.com/w/c/string/wide/iswalnum iswalnum] | checks whether the operand is alphanumeric | |
[http://en.cppreference.com/w/c/string/byte/isalpha isalpha] | [http://en.cppreference.com/w/c/string/wide/iswalpha iswalpha] | checks whether the operand is alphabetic | |
[http://en.cppreference.com/w/c/string/byte/islower islower] | [http://en.cppreference.com/w/c/string/wide/iswlower iswlower] | checks whether the operand is lowercase | |
[http://en.cppreference.com/w/c/string/byte/isupper isupper] | [http://en.cppreference.com/w/c/string/wide/iswupper iswupper] | checks whether the operand is an uppercase | |
[http://en.cppreference.com/w/c/string/byte/isdigit isdigit] | [http://en.cppreference.com/w/c/string/wide/iswdigit iswdigit] | checks whether the operand is a digit | |
[http://en.cppreference.com/w/c/string/byte/isxdigit isxdigit] | [http://en.cppreference.com/w/c/string/wide/iswxdigit iswxdigit] | checks whether the operand is hexadecimal | |
[http://en.cppreference.com/w/c/string/byte/iscntrl iscntrl] | [http://en.cppreference.com/w/c/string/wide/iswcntrl iswcntrl] | checks whether the operand is a control character | |
[http://en.cppreference.com/w/c/string/byte/isgraph isgraph] | [http://en.cppreference.com/w/c/string/wide/iswgraph iswgraph] | checks whether the operand is a graphical character | |
[http://en.cppreference.com/w/c/string/byte/isspace isspace] | [http://en.cppreference.com/w/c/string/wide/iswspace iswspace] | checks whether the operand is space | |
[http://en.cppreference.com/w/c/string/byte/isblank isblank] | [http://en.cppreference.com/w/c/string/wide/iswblank iswblank] | checks whether the operand is a blank space character | |
[http://en.cppreference.com/w/c/string/byte/isprint isprint] | [http://en.cppreference.com/w/c/string/wide/iswprint iswprint] | checks whether the operand is a printable character | |
[http://en.cppreference.com/w/c/string/byte/ispunct ispunct] | [http://en.cppreference.com/w/c/string/wide/iswpunct iswpunct] | checks whether the operand is punctuation | |
[http://en.cppreference.com/w/c/string/byte/tolower tolower] | [http://en.cppreference.com/w/c/string/wide/towlower towlower] | converts the operand to lowercase | |
[http://en.cppreference.com/w/c/string/byte/toupper toupper] | [http://en.cppreference.com/w/c/string/wide/towupper towupper] | converts the operand to uppercase | |
[http://en.cppreference.com/w/c/string/wide/iswctype iswctype] | checks whether the operand falls into specific class | ||
[http://en.cppreference.com/w/c/string/wide/towctrans towctrans] | converts the operand using a specific mapping | ||
[http://en.cppreference.com/w/c/string/wide/wctype wctype] | returns a wide character class to be used with iswctype | ||
[http://en.cppreference.com/w/c/string/wide/wctrans wctrans] | returns a transformation mapping to be used with towctrans |