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 PEKC(void) peekc code
#define UNGETC(void) ungetc code
#define RETURN(ptr) return code
#define EROR(val) error code
extern char *loc1, *loc2, *locs;
#include
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 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 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
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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
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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 spe-
cial 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 ASCI 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 com-
ponent of the RE.
2.5 A RE enclosed between the character sequences \( and
\) is a RE that matches whatever the unadorned RE
matches.
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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
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, 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
constrains 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 "$" addresses the last line of the
buffer.
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3. A decimal number n addresses the n-th line of the
buffer.
4. 'x addresses the line marked with the mark name
character x, which must be an ASCI lower-case
letter (a-z). Lines are marked with the k command
described below.
5. A RE enclosed by slashes (/) addresses the first
line found by searching forward from the line fol-
lowing the current line toward the end of the
buffer and stopping at the first line containing a
string matching the RE. If necessary, the search
wraps around to the beginning of the buffer and
continues up to and including the current line, so
that the entire buffer is searched.
6. A RE enclosed in question marks (?) addresses the
first line found by searching backward from the
line preceding the current line toward the begin-
ning of the buffer and stopping at the first line
containing a string matching the RE. If necessary,
the search wraps around to the end of the buffer
and continues up to and including the current line.
7. An address followed by a plus sign (]) or a minus
sign (-) followed by a decimal number specifies
that address plus (respectively minus) the indi-
cated number of lines. A shorthand for .]5 is .5.
8. If an address begins with ] or -, the addition or
subtraction is taken with respect to the current
line; for example, -5 is understood to mean .-5.
9. If an address ends with ] or -, then 1 is added to
or subtracted from the address, respectively. As a
consequence of this rule and of Rule 8, immediately
above, the address - refers to the line preceding
the current line. (To maintain compatibility with
earlier versions of the editor, the character ^ in
addresses is entirely equivalent to -.) Moreover,
trailing ] and - characters have a cumulative
effect, so -- refers to the current line less 2.
10. For convenience, a comma (,) stands for the address
pair 1,$, 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
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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 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 statement. These macros are used by
the compile() routine. The macros GETC, PEKC, 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.
PEKC This macro returns the next character (byte)
in the regular expression. Immediately suc-
cessive calls to PEKC 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 PEKC.
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.
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RETURN(ptr) This macro is used on normal exit of the com-
pile() 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.
EROR(val) This macro is the abnormal return from the
compile() routine. The argument val is an
error number (see ERORS 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 EROR(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 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, PEKC, and UNGETC.
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Otherwise it can be used to declare external variables that
might be used by GETC, PEKC 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.
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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 PEKC() (*sp)
#define UNGETC(c) (--sp)
#define RETURN(c) return;
#define EROR(c) regerr()
#include
. . .
(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 EROR 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 \(.
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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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