Below is a description of the format string as taken from the SunOs manual.

The control string usually contains conversion specifications, which are used to direct interpretation of input sequences. The control string may contain:

• White-space characters (SPACE, TAB, or NEWLINE) which, except in two cases described below, cause input to be read up to the next non-white-space character.
• An ordinary character (not _%_), which must match the next character of the input stream.
• Conversion specifications, consisting of the character _%_ or the character sequence %digit$, an optional assignment suppressing character _*_, an optional numerical maximum field width, an optional l (ell) or h indicating the size of the receiving variable, and a conversion code.

  Conversion specifications are introduced by the character % or the character sequence %digit$. A conversion specification directs the conversion of the next input field; the result is placed in the variable pointed to by the corresponding argument, unless assignment suppression was indicated by _*_. The suppression of assignment provides a way of describing an input field which is to be skipped. An input field is defined as a string of non-space characters; it extends to the next inappropriate character or until the field width, if specified, is exhausted. For all descriptors except _[_' and c’, white space leading an input field is ignored.

  The conversion character indicates the interpretation of the input field; the corresponding pointer argument must usually be of a restricted type. For a suppressed field, no pointer argument is given. The following conversion characters are legal:

  # %
  A single % is expected in the input at this point; no assignment
  is done.

  The various integer formats (the result is always returned as a PCE int).

• d A decimal integer is expected; the corresponding argument should be an integer pointer.
• u An unsigned decimal integer is expected; the corresponding argument should be an unsigned integer pointer.
• o An octal integer is expected; the corresponding argument should be an integer pointer.
• x A hexadecimal integer is expected; the corresponding argument should be an integer pointer.
• i An integer is expected; the corresponding argument should be an integer pointer. It will store the value of the next input item interpreted according to C conventions: a leading _0_' implies octal; a leading _0x_’implies hexadecimal; otherwise, decimal.
• n Stores in an integer argument the total number of characters (including white space) that have been scanned so far since the function call. No input is consumed.

  The various floating point formats. The result is passed as an instance of class real.

• e,f,g A floating point number is expected; the next field is converted accordingly and stored through the corresponding argument, which should be a pointer to a float. The input format for floating point numbers is as described for string_to_decimal(3), with fortran_conventions zero.

  A character. The result is returned as the ASCII (PCE int).

• c A character is expected; the corresponding argument should be a character pointer. The normal skip over white space is suppressed in this case; to read the next non-space character, use %1s. If a field width is given, the corresponding argument should refer to a character array, and the indicated number of characters is read.

  The string formats. The result is a PCE string object.

• s A character string is expected; the corresponding argument should be a character pointer pointing to an array of characters large enough to accept the string and a terminating \0, which will be added automatically. The input field is terminated by a white space character.

  # [ Indicates string data; the normal skip over leading white space is suppressed. The left bracket is followed by a set of characters, which we will call the scanset, and a right bracket; the input field is the maximal sequence of input characters consisting entirely of characters in the scanset. The circumflex (^), when it appears as the first character in the scanset, serves as a complement operator and redefines the scanset as the set of all characters not contained in the remainder of the scanset string. There are some conventions used in the construction of the scanset. A range of characters may be represented by the construct first-last, thus [0123456789] may be expressed [0-9]. Using this convention, first must be lexically less than or equal to last, or else the dash will stand for itself. The dash will also stand for itself whenever it is the first or the last character in the scanset. To include the right square bracket as an element of the scanset, it must appear as the first character (possibly pre- ceded by a circumflex) of the scanset, and in this case it will not be syntactically interpreted as the closing bracket. The corresponding argument must point to a character array large enough to hold the data field and the terminating \0, which will be added automatically. At least one character must match for this conversion to be considered successful.

  See also class regex.