Tribblix: manual page: regexp.7

REGEXP(7) Standards, Environments, and Macros REGEXP(7)

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(7) 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.