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(libc.info)Classification of Characters

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Classification of Characters
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   This section explains the library functions for classifying
characters.  For example, `isalpha' is the function to test for an
alphabetic character.  It takes one argument, the character to test,
and returns a nonzero integer if the character is alphabetic, and zero
otherwise.  You would use it like this:

     if (isalpha (c))
       printf ("The character `%c' is alphabetic.\n", c);

   Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an `int' which is treated as a boolean value.  The character
argument is passed as an `int', and it may be the constant value `EOF'
instead of a real character.

   The attributes of any given character can vary between locales.
Note: Locales, for more information on locales.

   These functions are declared in the header file `ctype.h'.

 - Function: int islower (int C)
     Returns true if C is a lower-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 - Function: int isupper (int C)
     Returns true if C is an upper-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 - Function: int isalpha (int C)
     Returns true if C is an alphabetic character (a letter).  If
     `islower' or `isupper' is true of a character, then `isalpha' is
     also true.

     In some locales, there may be additional characters for which
     `isalpha' is true--letters which are neither upper case nor lower
     case.  But in the standard `"C"' locale, there are no such
     additional characters.

 - Function: int isdigit (int C)
     Returns true if C is a decimal digit (`0' through `9').

 - Function: int isalnum (int C)
     Returns true if C is an alphanumeric character (a letter or
     number); in other words, if either `isalpha' or `isdigit' is true
     of a character, then `isalnum' is also true.

 - Function: int isxdigit (int C)
     Returns true if C is a hexadecimal digit.  Hexadecimal digits
     include the normal decimal digits `0' through `9' and the letters
     `A' through `F' and `a' through `f'.

 - Function: int ispunct (int C)
     Returns true if C is a punctuation character.  This means any
     printing character that is not alphanumeric or a space character.

 - Function: int isspace (int C)
     Returns true if C is a "whitespace" character.  In the standard
     `"C"' locale, `isspace' returns true for only the standard
     whitespace characters:

    `' ''
          space

    `'\f''
          formfeed

    `'\n''
          newline

    `'\r''
          carriage return

    `'\t''
          horizontal tab

    `'\v''
          vertical tab

 - Function: int isblank (int C)
     Returns true if C is a blank character; that is, a space or a tab.
     This function is a GNU extension.

 - Function: int isgraph (int C)
     Returns true if C is a graphic character; that is, a character
     that has a glyph associated with it.  The whitespace characters
     are not considered graphic.

 - Function: int isprint (int C)
     Returns true if C is a printing character.  Printing characters
     include all the graphic characters, plus the space (` ') character.

 - Function: int iscntrl (int C)
     Returns true if C is a control character (that is, a character that
     is not a printing character).

 - Function: int isascii (int C)
     Returns true if C is a 7-bit `unsigned char' value that fits into
     the US/UK ASCII character set.  This function is a BSD extension
     and is also an SVID extension.