Manual page for regexp(5)
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
#include <regexp.h>
char *compile(char *instring, char *expbuf, char *endbuf,
int eof);
int step(char *string, char *expbuf);
int advance(char *string, char *expbuf);
extern char *loc1, *loc2, *locs;
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:
-
- only Basic Regular Expressions are supported
- 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.
A 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
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
Created by unroff & hp-tools.
© by Hans-Peter Bischof. All Rights Reserved (1997).
Last modified 21/April/97
