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.\"     @(#)regexp.3	5.2 (Berkeley) 4/20/91 
.\" 
.Dd April 20, 1991 
.Dt REGEXP 3 
.Os 
.Sh NAME 
.Nm regcomp , 
.Nm regexec , 
.Nm regsub , 
.Nm regerror 
.Nd regular expression handlers 
.Sh SYNOPSIS 
.Fd #include  
.Ft regexp * 
.Fn regcomp "const char *exp" 
.Ft int 
.Fn regexec "const regexp *prog" "const char *string" 
.Ft void 
.Fn regsub "const regexp *prog" "const char *source" "char *dest" 
.Sh DESCRIPTION 
The 
.Fn regcomp , 
.Fn regexec , 
.Fn regsub , 
and 
.Fn regerror 
functions 
implement 
.Xr egrep 1 Ns -style 
regular expressions and supporting facilities. 
.Pp 
The 
.Fn regcomp 
function 
compiles a regular expression into a structure of type 
.Xr regexp , 
and returns a pointer to it. 
The space has been allocated using 
.Xr malloc 3 
and may be released by 
.Xr free . 
.Pp 
The 
.Fn regexec 
function 
matches a 
.Dv NUL Ns -terminated 
.Fa string 
against the compiled regular expression 
in 
.Fa prog . 
It returns 1 for success and 0 for failure, and adjusts the contents of 
.Fa prog Ns 's 
.Em startp 
and 
.Em endp 
(see below) accordingly. 
.Pp 
The members of a 
.Xr regexp 
structure include at least the following (not necessarily in order): 
.Bd -literal -offset indent 
char *startp[NSUBEXP]; 
char *endp[NSUBEXP]; 
.Ed 
.Pp 
where 
.Dv NSUBEXP 
is defined (as 10) in the header file. 
Once a successful 
.Fn regexec 
has been done using the 
.Fn regexp , 
each 
.Em startp Ns - Em endp 
pair describes one substring 
within the 
.Fa string , 
with the 
.Em startp 
pointing to the first character of the substring and 
the 
.Em endp 
pointing to the first character following the substring. 
The 0th substring is the substring of 
.Fa string 
that matched the whole 
regular expression. 
The others are those substrings that matched parenthesized expressions 
within the regular expression, with parenthesized expressions numbered 
in left-to-right order of their opening parentheses. 
.Pp 
The 
.Fn regsub 
function 
copies 
.Fa source 
to 
.Fa dest , 
making substitutions according to the 
most recent 
.Fn regexec 
performed using 
.Fa prog . 
Each instance of `&' in 
.Fa source 
is replaced by the substring 
indicated by 
.Em startp Ns Bq 
and 
.Em endp Ns Bq . 
Each instance of 
.Sq \e Ns Em n , 
where 
.Em n 
is a digit, is replaced by 
the substring indicated by 
.Em startp Ns Bq Em n 
and 
.Em endp Ns Bq Em n . 
To get a literal `&' or 
.Sq \e Ns Em n 
into 
.Fa dest , 
prefix it with `\e'; 
to get a literal `\e' preceding `&' or 
.Sq \e Ns Em n , 
prefix it with 
another `\e'. 
.Pp 
The 
.Fn regerror 
function 
is called whenever an error is detected in 
.Fn regcomp , 
.Fn regexec , 
or 
.Fn regsub . 
The default 
.Fn regerror 
writes the string 
.Fa msg , 
with a suitable indicator of origin, 
on the standard 
error output 
and invokes 
.Xr exit 2 . 
The 
.Fn regerror 
function 
can be replaced by the user if other actions are desirable. 
.Sh REGULAR EXPRESSION SYNTAX 
A regular expression is zero or more 
.Em branches , 
separated by `|'. 
It matches anything that matches one of the branches. 
.Pp 
A branch is zero or more 
.Em pieces , 
concatenated. 
It matches a match for the first, followed by a match for the second, etc. 
.Pp 
A piece is an 
.Em atom 
possibly followed by `*', `+', or `?'. 
An atom followed by `*' matches a sequence of 0 or more matches of the atom. 
An atom followed by `+' matches a sequence of 1 or more matches of the atom. 
An atom followed by `?' matches a match of the atom, or the null string. 
.Pp 
An atom is a regular expression in parentheses (matching a match for the 
regular expression), a 
.Em range 
(see below), `.' 
(matching any single character), `^' (matching the null string at the 
beginning of the input string), `$' (matching the null string at the 
end of the input string), a `\e' followed by a single character (matching 
that character), or a single character with no other significance 
(matching that character). 
.Pp 
A 
.Em range 
is a sequence of characters enclosed in `[]'. 
It normally matches any single character from the sequence. 
If the sequence begins with `^', 
it matches any single character 
.Em not 
from the rest of the sequence. 
If two characters in the sequence are separated by `\-', this is shorthand 
for the full list of 
.Tn ASCII 
characters between them 
(e.g. `[0-9]' matches any decimal digit). 
To include a literal `]' in the sequence, make it the first character 
(following a possible `^'). 
To include a literal `\-', make it the first or last character. 
.Sh AMBIGUITY 
If a regular expression could match two different parts of the input string, 
it will match the one which begins earliest. 
If both begin in the same place but match different lengths, or match 
the same length in different ways, life gets messier, as follows. 
.Pp 
In general, the possibilities in a list of branches are considered in 
left-to-right order, the possibilities for `*', `+', and `?' are 
considered longest-first, nested constructs are considered from the 
outermost in, and concatenated constructs are considered leftmost-first. 
The match that will be chosen is the one that uses the earliest 
possibility in the first choice that has to be made. 
If there is more than one choice, the next will be made in the same manner 
(earliest possibility) subject to the decision on the first choice. 
And so forth. 
.Pp 
For example, 
.Sq Li (ab|a)b*c 
could match 
`abc' in one of two ways. 
The first choice is between `ab' and `a'; since `ab' is earlier, and does 
lead to a successful overall match, it is chosen. 
Since the `b' is already spoken for, 
the `b*' must match its last possibility\(emthe empty string\(emsince 
it must respect the earlier choice. 
.Pp 
In the particular case where no `|'s are present and there is only one 
`*', `+', or `?', the net effect is that the longest possible 
match will be chosen. 
So 
.Sq Li ab* , 
presented with `xabbbby', will match `abbbb'. 
Note that if 
.Sq Li ab* , 
is tried against `xabyabbbz', it 
will match `ab' just after `x', due to the begins-earliest rule. 
(In effect, the decision on where to start the match is the first choice 
to be made, hence subsequent choices must respect it even if this leads them 
to less-preferred alternatives.) 
.Sh RETURN VALUES 
The 
.Fn regcomp 
function 
returns 
.Dv NULL 
for a failure 
.Pf ( Fn regerror 
permitting), 
where failures are syntax errors, exceeding implementation limits, 
or applying `+' or `*' to a possibly-null operand. 
.Sh SEE ALSO 
.Xr ed 1 , 
.Xr ex 1 , 
.Xr expr 1 , 
.Xr egrep 1 , 
.Xr fgrep 1 , 
.Xr grep 1 , 
.Xr regex 3 
.Sh HISTORY 
Both code and manual page for 
.Fn regcomp , 
.Fn regexec , 
.Fn regsub , 
and 
.Fn regerror 
were written at the University of Toronto 
and appeared in 
.Bx 4.3 tahoe . 
They are intended to be compatible with the Bell V8 
.Xr regexp 3 , 
but are not derived from Bell code. 
.Sh BUGS 
Empty branches and empty regular expressions are not portable to V8. 
.Pp 
The restriction against 
applying `*' or `+' to a possibly-null operand is an artifact of the 
simplistic implementation. 
.Pp 
Does not support 
.Xr egrep Ns 's 
newline-separated branches; 
neither does the V8 
.Xr regexp 3 , 
though. 
.Pp 
Due to emphasis on 
compactness and simplicity, 
it's not strikingly fast. 
It does give special attention to handling simple cases quickly.