REGEXP(5) Standards, Environments, and Macros REGEXP(5)


NAME


regexp, compile, step, advance - simple regular expression compile and
match routines

SYNOPSIS


#define INIT declarations
#define GETC(void) getc code
#define PEEKC(void) peekc code
#define UNGETC(void) ungetc code
#define RETURN(ptr) return code
#define ERROR(val) error code

extern char *loc1, *loc2, *locs;

#include <regexp.h>

char *compile(char *instring, char *expbuf, const char *endfug, int eof);


int step(const char *string, const char *expbuf);


int advance(const char *string, const char *expbuf);


DESCRIPTION


Regular Expressions (REs) provide a mechanism to select specific strings
from a set of character strings. The Simple Regular Expressions described
below differ from the Internationalized Regular Expressions described on
the regex(5) manual page in the following ways:

o only Basic Regular Expressions are supported

o the Internationalization features--character class,
equivalence class, and multi-character collation--are not
supported.


The functions step(), advance(), and compile() are general purpose
regular expression matching routines to be used in programs that perform
regular expression matching. These functions are defined by the
<regexp.h> header.


The functions step() and advance() do pattern matching given a character
string and a compiled regular expression as input.


The function compile() takes as input a regular expression as defined
below and produces a compiled expression that can be used with step() or
advance().

Basic Regular Expressions


A regular expression specifies a set of character strings. A member of
this set of strings is said to be matched by the regular expression. Some
characters have special meaning when used in a regular expression; other
characters stand for themselves.


The following one-character REs match a single character:

1.1
An ordinary character ( not one of those discussed in 1.2 below)
is a one-character RE that matches itself.


1.2
A backslash (\) followed by any special character is a one-
character RE that matches the special character itself. The
special characters are:

a.
., *, [, and \ (period, asterisk, left square bracket, and
backslash, respectively), which are always special, except
when they appear within square brackets ([]; see 1.4 below).


b.
^ (caret or circumflex), which is special at the beginning
of an entire RE (see 4.1 and 4.3 below), or when it
immediately follows the left of a pair of square brackets
([]) (see 1.4 below).


c.
$ (dollar sign), which is special at the end of an entire RE
(see 4.2 below).


d.
The character used to bound (that is, delimit) an entire RE,
which is special for that RE (for example, see how slash (/)
is used in the g command, below.)


1.3
A period (.) is a one-character RE that matches any character
except new-line.


1.4
A non-empty string of characters enclosed in square brackets ([])
is a one-character RE that matches any one character in that
string. If, however, the first character of the string is a
circumflex (^), the one-character RE matches any character except
new-line and the remaining characters in the string. The ^ has
this special meaning only if it occurs first in the string. The
minus (-) may be used to indicate a range of consecutive
characters; for example, [0-9] is equivalent to [0123456789]. The
- loses this special meaning if it occurs first (after an initial
^, if any) or last in the string. The right square bracket (])
does not terminate such a string when it is the first character
within it (after an initial ^, if any); for example, []a-f]
matches either a right square bracket (]) or one of the ASCII
letters a through f inclusive. The four characters listed in 1.2.a
above stand for themselves within such a string of characters.


The following rules may be used to construct REs from one-character REs:

2.1
A one-character RE is a RE that matches whatever the one-character
RE matches.


2.2
A one-character RE followed by an asterisk (*) is a RE that
matches 0 or more occurrences of the one-character RE. If there is
any choice, the longest leftmost string that permits a match is
chosen.


2.3
A one-character RE followed by \{m\}, \{m,\}, or \{m,n\} is a RE
that matches a range of occurrences of the one-character RE. The
values of m and n must be non-negative integers less than 256;
\{m\} matches exactly m occurrences; \{m,\} matches at least m
occurrences; \{m,n\} matches any number of occurrences between m
and n inclusive. Whenever a choice exists, the RE matches as many
occurrences as possible.


2.4
The concatenation of REs is a RE that matches the concatenation of
the strings matched by each component of the RE.


2.5
A RE enclosed between the character sequences \( and \) is a RE
that matches whatever the unadorned RE matches.


2.6
The expression \n matches the same string of characters as was
matched by an expression enclosed between \( and \) earlier in the
same RE. Here n is a digit; the sub-expression specified is that
beginning with the n-th occurrence of \( counting from the left.
For example, the expression ^\(.*\)\1$ matches a line consisting
of two repeated appearances of the same string.


An RE may be constrained to match words.

3.1
\< constrains a RE to match the beginning of a string or to follow
a character that is not a digit, underscore, or letter. The first
character matching the RE must be a digit, underscore, or letter.


3.2
\> constrains a RE to match the end of a string or to precede a
character that is not a digit, underscore, or letter.


An entire RE may be constrained to match only an initial segment or final
segment of a line (or both).

4.1
A circumflex (^) at the beginning of an entire RE constrains that
RE to match an initial segment of a line.


4.2
A dollar sign ($) at the end of an entire RE constrains that RE to
match a final segment of a line.


4.3
The construction ^entire RE$ constrains the entire RE to match the
entire line.


The null RE (for example, //) is equivalent to the last RE encountered.

Addressing with REs


Addresses are constructed as follows:

1. The character "." addresses the current line.

2. The character "$" addresses the last line of the buffer.

3. A decimal number n addresses the n-th line of the buffer.

4. 'x addresses the line marked with the mark name character x,
which must be an ASCII lower-case letter (a-z). Lines are
marked with the k command described below.

5. A RE enclosed by slashes (/) addresses the first line found by
searching forward from the line following the current line
toward the end of the buffer and stopping at the first line
containing a string matching the RE. If necessary, the search
wraps around to the beginning of the buffer and continues up
to and including the current line, so that the entire buffer
is searched.

6. A RE enclosed in question marks (?) addresses the first line
found by searching backward from the line preceding the
current line toward the beginning of the buffer and stopping
at the first line containing a string matching the RE. If
necessary, the search wraps around to the end of the buffer
and continues up to and including the current line.

7. An address followed by a plus sign (+) or a minus sign (-)
followed by a decimal number specifies that address plus
(respectively minus) the indicated number of lines. A
shorthand for .+5 is .5.

8. If an address begins with + or -, the addition or subtraction
is taken with respect to the current line; for example, -5 is
understood to mean .-5.

9. If an address ends with + or -, then 1 is added to or
subtracted from the address, respectively. As a consequence of
this rule and of Rule 8, immediately above, the address -
refers to the line preceding the current line. (To maintain
compatibility with earlier versions of the editor, the
character ^ in addresses is entirely equivalent to -.)
Moreover, trailing + and - characters have a cumulative
effect, so -- refers to the current line less 2.

10. For convenience, a comma (,) stands for the address pair 1,$,
while a semicolon (;) stands for the pair .,$.

Characters With Special Meaning


Characters that have special meaning except when they appear within
square brackets ([]) or are preceded by \ are: ., *, [, \. Other special
characters, such as $ have special meaning in more restricted contexts.


The character ^ at the beginning of an expression permits a successful
match only immediately after a newline, and the character $ at the end of
an expression requires a trailing newline.


Two characters have special meaning only when used within square
brackets. The character - denotes a range, [c-c], unless it is just after
the open bracket or before the closing bracket, [-c] or [c-] in which
case it has no special meaning. When used within brackets, the character
^ has the meaning complement of if it immediately follows the open
bracket (example: [^c]); elsewhere between brackets (example: [c^]) it
stands for the ordinary character ^.


The special meaning of the \ operator can be escaped only by preceding it
with another \, for example \\.

Macros


Programs must have the following five macros declared before the #include
<regexp.h> statement. These macros are used by the compile() routine.
The macros GETC, PEEKC, and UNGETC operate on the regular expression
given as input to compile().

GETC
This macro returns the value of the next character (byte)
in the regular expression pattern. Successive calls to
GETC should return successive characters of the regular
expression.


PEEKC
This macro returns the next character (byte) in the
regular expression. Immediately successive calls to
PEEKC should return the same character, which should also
be the next character returned by GETC.


UNGETC
This macro causes the argument c to be returned by the
next call to GETC and PEEKC. No more than one character of
pushback is ever needed and this character is guaranteed
to be the last character read by GETC. The return value of
the macro UNGETC(c) is always ignored.


RETURN(ptr)
This macro is used on normal exit of the compile()
routine. The value of the argument ptr is a pointer to the
character after the last character of the compiled regular
expression. This is useful to programs which have memory
allocation to manage.


ERROR(val)
This macro is the abnormal return from the compile()
routine. The argument val is an error number (see ERRORS
below for meanings). This call should never return.


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

compile(instring, expbuf, endbuf, eof)


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 which 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
place where the compiled regular expression will be placed.


The parameter endbuf is one more than the highest address where the
compiled regular expression may 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. This character is usually a /.


Each program that includes the <regexp.h> header file must have a #define
statement for INIT. 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 EXAMPLES below.)

step(), advance()
The first parameter to the step() and advance() functions 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 which
was obtained by a call to the function compile().


The function step() returns non-zero if some substring of string matches
the regular expression in expbuf and 0 if there is no match. If there is
a match, two external character pointers are set as a side effect to the
call to step(). The variable loc1 points to the first character that
matched the regular expression; the variable loc2 points to the character
after the last character that matches the regular expression. Thus if
the regular expression matches the entire input string, loc1 will point
to the first character of string and loc2 will point to the null at the
end of string.


The function advance() returns non-zero if the initial substring of
string matches the regular expression in expbuf. If there is a match, an
external character pointer, loc2, is set as a side effect. The variable
loc2 points to the next character in string after the last character that
matched.


When advance() encounters a * or \{ \} sequence in the regular
expression, it will advance its pointer to the string to be matched as
far as possible and will recursively call itself trying to match the rest
of the string to the rest of the regular expression. As long as there is
no match, advance() will back 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() will break out of the loop that backs up and will
return zero.


The external variables circf, sed, and nbra are reserved.

EXAMPLES


Example 1: Using Regular Expression Macros and Calls




The following is an example of how the regular expression macros and
calls might be defined by an application program:


#define INIT register char *sp = instring;
#define GETC() (*sp++)
#define PEEKC() (*sp)
#define UNGETC(c) (--sp)
#define RETURN(c) return;
#define ERROR(c) regerr()

#include <regexp.h>
...
(void) compile(*argv, expbuf, &expbuf[ESIZE],'\0');
...
if (step(linebuf, expbuf))
succeed;


DIAGNOSTICS


The function compile() uses the macro RETURN on success and the macro
ERROR on failure (see above). The functions step() and advance() return
non-zero on a successful match and zero if there is no match. Errors are:

11
range endpoint too large.


16
bad number.


25
\ digit out of range.


36
illegal or missing delimiter.


41
no remembered search string.


42
\( \) imbalance.


43
too many \(.


44
more than 2 numbers given in \{ \}.


45
} expected after \.


46
first number exceeds second in \{ \}.


49
[ ] imbalance.


50
regular expression overflow.


SEE ALSO


regex(5)


May 20, 2002 REGEXP(5)