4.3BSD-Reno/share/man/cat3/regexec.0
REGEXP(3) 1988 REGEXP(3)
NNAAMMEE
regcomp, regexec, regsub, regerror - regular expression
handlers
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;;
NNAAMMEE
_R_e_g_c_o_m_p, _r_e_g_e_x_e_c, _r_e_g_s_u_b, and _r_e_g_e_r_r_o_r 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 fol-
lowing (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. 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 sub-
string and the _e_n_d_p pointing to the first character follow-
ing the substring. The 0th substring is the substring of
_s_t_r_i_n_g that matched the whole regular expression. The oth-
ers are those substrings that matched parenthesized expres-
sions within the regular expression, with parenthesized
expressions numbered in left-to-right order of their opening
parentheses.
Printed 7/27/90 May 1
REGEXP(3) 1988 REGEXP(3)
_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 indi-
cated 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 end of the input string), a `\' fol-
lowed 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 charac-
ter _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
Printed 7/27/90 May 2
REGEXP(3) 1988 REGEXP(3)
earliest. If both begin in the same place but match dif-
ferent lengths, or match the same length in different ways,
life gets messier, as follows.
In general, the possibilities in a list of branches are con-
sidered in left-to-right order, the possibilities for `*',
`+', and `?' are considered longest-first, nested constructs
are considered from the outermost in, and concatenated con-
structs 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 (earli-
est 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 deci-
sion 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.)
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 for _r_e_g_c_o_m_p, _r_e_g_e_x_e_c, _r_e_g_s_u_b, and
_r_e_g_e_r_r_o_r were written at the University of Toronto. 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 port-
able to V8.
The restriction against applying `*' or `+' to a possibly-
null operand is an artifact of the simplistic implementa-
tion.
Printed 7/27/90 May 3
REGEXP(3) 1988 REGEXP(3)
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.
SSEEEE AALLSSOO
ed(1), ex(1), expr(1), egrep(1), fgrep(1), grep(1), regex(3)
Printed 7/27/90 May 4