regexp(3X)

DESCRIPTION

These functions are general-purpose regular expression matching routines to be used in programs that perform Basic Regular Expression (see regexp(5)) matching. These functions are defined in <regexp.h>.

The functions step() and advance() do pattern matching given a character string and a compiled regular expression as input. compile() takes a Basic Regular Expression as input and produces a compiled expression that can be used with step() and advance().

The interface to this file is unpleasantly complex. Programs that include this file must have the following five macros declared before the #include <regexp.h> statement. These macros are used by the compile() routine.

The syntax of the compile() routine is as follows:

The first parameter instring is never used explicitly by the compile() routine, but is useful for programs that pass down different pointers to input characters. It is sometimes used in the INIT declaration (see below). Programs that call functions to input characters or have characters in an external array can pass down a value of ((char *) 0) for this parameter.

The next parameter expbuf is a character pointer. It points to the location where the compiled regular expression will be placed.

The parameter endbuf is one more than the highest address where the compiled regular expression can be placed. If the compiled expression cannot fit in (endbuf-expbuf) bytes, a call to ERROR(50) is made.

The parameter eof is the character which marks the end of the regular expression. For example, in ed(1), this character is usually a /.

Each program that includes this file must have a #define statement for INIT. This definition is placed right after the declaration for the function compile() and the opening curly brace {. It is used for dependent declarations and initializations. Most often it is used to set a register variable to point to the beginning of the regular expression so that this register variable can be used in the declarations for GETC(), PEEKC(), and UNGETC(). Otherwise it can be used to declare external variables that might be used by GETC(), PEEKC(), and UNGETC(). See the example below of the declarations taken from grep(1).

step() also performs actual regular expression matching in this file. The call to step is as follows:

The first parameter to step() is a pointer to a string of characters to be checked for a match. This string should be null-terminated.

The second parameter expbuf is the compiled regular expression that was obtained by a call to compile().

step() returns non-zero if the given string matches the regular expression, and zero if the expressions do not match. If there is a match, two external character pointers are set as a side effect to the call to step(). The variable set in step() is loc1. This is a pointer to the first character that matched the regular expression. The variable loc2, which is set by the function advance(), points to the character after the last character that matches the regular expression. Thus, if the regular expression matches the entire line, loc1 points to the first character of string and loc2 points to the null at the end of string.

step() uses the external variable circf, which is set by compile() if the regular expression begins with ^. If this is set, step() tries to match the regular expression to the beginning of the string only. If more than one regular expression is to be compiled before the first is executed, the value of circf should be saved for each compiled expression and circf should be set to that saved value before each call to step().

advance() is called from step() with the same arguments as step(). The purpose of step() is to step through the string argument and call advance() until advance() returns non-zero, which indicates a match, or until the end of string is reached. To constrain string to beginning-of-line in all cases, step() need not be called; simply call advance().

When advance() encounters a * or \{\} sequence in the regular expression, it advances its pointer to the string to be matched as far as possible and recursively calls itself, trying to match the rest of the string to the rest of the regular expression. As long as there is no match, advance backs up along the string until it finds a match or reaches the point in the string that initially matched the * or \{\}. It is sometimes desirable to stop this backing up before the initial point in the string is reached. If the external character pointer locs is equal to the point in the string at sometime during the backing up process, advance() breaks out of the loop that backs up and returns zero. This is used by ed(1) and sed(1) for substitutions done globally (not just the first occurrence, but the whole line) so, for example, expressions such as s/y*//g do not loop forever.

The additional external variables sed and nbra are used for special purposes.

EXTERNAL INFLUENCES

EXAMPLES

The following is an example of how the regular expression macros and calls look from grep(1):

SEE ALSO

grep(1), regcomp(3C), setlocale(3C), regexp(5).

STANDARDS CONFORMANCE

advance(): AES, SVID2, SVID3, XPG2, XPG3, XPG4

compile(): AES, SVID2, SVID3, XPG2, XPG3, XPG4

loc1: AES, SVID2, SVID3, XPG2, XPG3, XPG4

loc2: AES, SVID2, SVID3, XPG2, XPG3, XPG4

locs: AES, SVID2, SVID3, XPG2, XPG3, XPG4

step(): AES, SVID2, SVID3, XPG2, XPG3, XPG4