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REGEXP(3) Library Functions REGEXP(3)
NNAAMMEE
regcomp, regexec, regsub, regerror - regular expression
handler
SSYYNNOOPPSSIISS
##iinncclluuddee <<rreeggeexxpp..hh>>
rreeggeexxpp **rreeggccoommpp((eexxpp))
cchhaarr **eexxpp;;
iinntt rreeggeexxeecc((pprroogg,, ssttrriinngg))
rreeggeexxpp **pprroogg;;
cchhaarr **ssttrriinngg;;
rreeggssuubb((pprroogg,, ssoouurrccee,, ddeesstt))
rreeggeexxpp **pprroogg;;
cchhaarr **ssoouurrccee;;
cchhaarr **ddeesstt;;
rreeggeerrrroorr((mmssgg))
cchhaarr **mmssgg;;
DDEESSCCRRIIPPTTIIOONN
These functions implement
_e_g_r_e_p(1)-style
regular expressions and supporting facilities.
_R_e_g_c_o_m_p
compiles a regular expression into a structure of type
_r_e_g_e_x_p,
and returns a pointer to it.
The space has been allocated using
_m_a_l_l_o_c(3)
and may be released by
_f_r_e_e.
_R_e_g_e_x_e_c
matches a NUL-terminated _s_t_r_i_n_g against the compiled regular expression
in _p_r_o_g.
It returns 1 for success and 0 for failure, and adjusts the contents of
_p_r_o_g's _s_t_a_r_t_p and _e_n_d_p (see below) accordingly.
The members of a
_r_e_g_e_x_p
structure include at least the following (not necessarily in order):
char *startp[NSUBEXP];
char *endp[NSUBEXP];
where
_N_S_U_B_E_X_P
is defined (as 10) in the header file.
Formatted 88/12/30 local 1
REGEXP(3) Library Functions REGEXP(3)
Once a successful _r_e_g_e_x_e_c has been done using the _r_e_g_e_x_p,
each _s_t_a_r_t_p-_e_n_d_p pair describes one substring
within the _s_t_r_i_n_g,
with the _s_t_a_r_t_p pointing to the first character of the substring and
the _e_n_d_p pointing to the first character following the substring.
The 0th substring is the substring of _s_t_r_i_n_g 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.
_R_e_g_s_u_b
copies _s_o_u_r_c_e to _d_e_s_t, making substitutions according to the
most recent _r_e_g_e_x_e_c performed using _p_r_o_g.
Each instance of `&' in _s_o_u_r_c_e is replaced by the substring
indicated by _s_t_a_r_t_p[_0] and
_e_n_d_p[_0].
Each instance of `\_n', where _n is a digit, is replaced by
the substring indicated by
_s_t_a_r_t_p[_n] and
_e_n_d_p[_n].
To get a literal `&' or `\_n' into _d_e_s_t, prefix it with `\';
to get a literal `\' preceding `&' or `\_n', prefix it with
another `\'.
_R_e_g_e_r_r_o_r
is called whenever an error is detected in _r_e_g_c_o_m_p, _r_e_g_e_x_e_c,
or _r_e_g_s_u_b.
The default _r_e_g_e_r_r_o_r writes the string _m_s_g,
with a suitable indicator of origin,
on the standard
error output
and invokes _e_x_i_t(2).
_R_e_g_e_r_r_o_r
can be replaced by the user if other actions are desirable.
RREEGGUULLAARR EEXXPPRREESSSSIIOONN SSYYNNTTAAXX
A regular expression is zero or more _b_r_a_n_c_h_e_s, separated by `|'.
It matches anything that matches one of the branches.
A branch is zero or more _p_i_e_c_e_s, concatenated.
It matches a match for the first, followed by a match for the second, etc.
A piece is an _a_t_o_m 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.
An atom is a regular expression in parentheses (matching a match for the
regular expression), a _r_a_n_g_e (see below), `.'
(matching any single character), `^' (matching the null string at the
beginning of the input string), `$' (matching the null string at the
Formatted 88/12/30 local 2
REGEXP(3) Library Functions REGEXP(3)
end of the input string), a `\' followed by a single character (matching
that character), or a single character with no other significance
(matching that character).
A _r_a_n_g_e 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 _n_o_t from the rest of the sequence.
If two characters in the sequence are separated by `-', this is shorthand
for the full list of 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.
AAMMBBIIGGUUIITTYY
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.
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.
For example, `(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--the empty string--since
it must respect the earlier choice.
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 `ab*', presented with `xabbbby', will match `abbbb'.
Note that if `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.)
SSEEEE AALLSSOO
egrep(1), expr(1)
Formatted 88/12/30 local 3
REGEXP(3) Library Functions REGEXP(3)
DDIIAAGGNNOOSSTTIICCSS
_R_e_g_c_o_m_p returns NULL for a failure
(_r_e_g_e_r_r_o_r permitting),
where failures are syntax errors, exceeding implementation limits,
or applying `+' or `*' to a possibly-null operand.
HHIISSTTOORRYY
Both code and manual page were
written at U of T.
They are intended to be compatible with the Bell V8 _r_e_g_e_x_p(3),
but are not derived from Bell code.
BBUUGGSS
Empty branches and empty regular expressions are not portable to V8.
The restriction against
applying `*' or `+' to a possibly-null operand is an artifact of the
simplistic implementation.
Does not support _e_g_r_e_p's newline-separated branches;
neither does the V8 _r_e_g_e_x_p(3), though.
Due to emphasis on
compactness and simplicity,
it's not strikingly fast.
It does give special attention to handling simple cases quickly.
Formatted 88/12/30 local 4