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 *end-
fug, 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 Inter-
nationalized 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 com-
mand, 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 con-
secutive 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 them-
selves 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 con-
catenation 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 charac-
ters 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 con-
sisting 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, under-
score, 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 con-
strains that RE to match an initial segment of a line.
4.2 A dollar sign ($) at the end of an entire RE con-
strains that RE to match a final segment of a line.
4.3 The construction ^entire RE$ 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 "$" addresses the last line of the buffer.
3. A decimal number n addresses the n-th line of the buffer.
4. 'x addresses the line marked with the mark name character
x, which must be an ASCII lower-case letter (a-z). Lines
are marked with the k command described below.
5. A RE enclosed by slashes (/) addresses the first line
found by searching forward from the line following the
current line toward the end of the buffer and stopping at
the first line containing a string matching the RE. If
necessary, the search wraps around to the beginning of
the buffer and continues up to and including the current
line, so that the entire buffer is searched.
6. A RE enclosed in question marks (?) addresses the first
line found by searching backward from the line preceding
the current line toward the beginning of the buffer and
stopping at the first line containing a string matching
the RE. If necessary, the search wraps around to the end
of the buffer and continues up to and including the
current line.
7. An address followed by a plus sign (+) or a minus sign
(-) followed by a decimal number specifies that address
plus (respectively minus) the indicated number of lines.
A shorthand for .+5 is .5.
8. If an address begins with + or -, the addition or sub-
traction is taken with respect to the current line; for
example, -5 is understood to mean .-5.
9. If an address ends with + or -, then 1 is added to or
subtracted from the address, respectively. As a conse-
quence of this rule and of Rule 8, immediately above, the
address - refers to the line preceding the current line.
(To maintain compatibility with earlier versions of the
editor, the character ^ in addresses is entirely
equivalent to -.) Moreover, trailing + and - characters
have a cumulative effect, so -- refers to the current
line less 2.
10.
For convenience, a comma (,) stands for the address pair
1,$, 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 mean-
ing 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 clos-
ing 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 (exam-
ple: [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 com-
pile().
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 charac-
ter 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. Other-
wise 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 com-
pile().
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 charac-
ter 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 sub-
string 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 regu-
lar 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: 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)
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