4.4BSD/usr/share/man/cat7/re_format.0
RE_FORMAT(7) BSD Reference Manual RE_FORMAT(7)
NNAAMMEE
re_format - POSIX 1003.2 regular expressions
DDEESSCCRRIIPPTTIIOONN
Regular expressions (``RE''s), as defined in POSIX 1003.2,
come in two forms: modern REs (roughly those of _e_g_r_e_p;
1003.2 calls these ``extended'' REs) and obsolete REs
(roughly those of _e_d; 1003.2 ``basic'' REs). Obsolete REs
mostly exist for backward compatibility in some old pro-
grams; they will be discussed at the end. 1003.2 leaves
some aspects of RE syntax and semantics open; `|-' marks
decisions on these aspects that may not be fully portable
to other 1003.2 implementations.
A (modern) RE is one|- or more non-empty|- _b_r_a_n_c_h_e_s, sepa-
rated by `|'. It matches anything that matches one of the
branches.
A branch is one|- 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 a single|- `*',
`+', `?', or _b_o_u_n_d. An atom followed by `*' matches a
sequence of 0 or more matches of the atom. An atom fol-
lowed by `+' matches a sequence of 1 or more matches of
the atom. An atom followed by `?' matches a sequence of 0
or 1 matches of the atom.
A _b_o_u_n_d is `{' followed by an unsigned decimal integer,
possibly followed by `,' possibly followed by another
unsigned decimal integer, always followed by `}'. The
integers must lie between 0 and RE_DUP_MAX (255|-) inclu-
sive, and if there are two of them, the first may not
exceed the second. An atom followed by a bound containing
one integer _i and no comma matches a sequence of exactly _i
matches of the atom. An atom followed by a bound contain-
ing one integer _i and a comma matches a sequence of _i or
more matches of the atom. An atom followed by a bound
containing two integers _i and _j matches a sequence of _i
through _j (inclusive) matches of the atom.
An atom is a regular expression enclosed in `()' (matching
a match for the regular expression), an empty set of `()'
(matching the null string)|-, a _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n (see
below), `.' (matching any single character), `^' (match-
ing the null string at the beginning of a line), `$'
(matching the null string at the end of a line), a `\'
followed by one of the characters `^.[$()|*+?{\' (matching
that character taken as an ordinary character), a `\' fol-
lowed by any other character|- (matching that character
4.4BSD June 4, 1993 1
RE_FORMAT(7) BSD Reference Manual RE_FORMAT(7)
taken as an ordinary character, as if the `\' had not been
present|-), or a single character with no other signifi-
cance (matching that character). A `{' followed by a
character other than a digit is an ordinary character, not
the beginning of a bound|-. It is illegal to end an RE
with `\'.
A _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n is a list of characters enclosed in
`[]'. It normally matches any single character from the
list (but see below). If the list begins with `^', it
matches any single character (but see below) _n_o_t from the
rest of the list. If two characters in the list are sepa-
rated by `-', this is shorthand for the full _r_a_n_g_e of
characters between those two (inclusive) in the collating
sequence, e.g. `[0-9]' in ASCII matches any decimal digit.
It is illegal|- for two ranges to share an endpoint, e.g.
`a-c-e'. Ranges are very collating-sequence-dependent,
and portable programs should avoid relying on them.
To include a literal `]' in the list, make it the first
character (following a possible `^'). To include a lit-
eral `-', make it the first or last character, or the sec-
ond endpoint of a range. To use a literal `-' as the
first endpoint of a range, enclose it in `[.' and `.]' to
make it a collating element (see below). With the excep-
tion of these and some combinations using `[' (see next
paragraphs), all other special characters, including `\',
lose their special significance within a bracket expres-
sion.
Within a bracket expression, a collating element (a char-
acter, a multi-character sequence that collates as if it
were a single character, or a collating-sequence name for
either) enclosed in `[.' and `.]' stands for the sequence
of characters of that collating element. The sequence is
a single element of the bracket expression's list. A
bracket expression containing a multi-character collating
element can thus match more than one character, e.g. if
the collating sequence includes a `ch' collating element,
then the RE `[[.ch.]]*c' matches the first five characters
of `chchcc'.
Within a bracket expression, a collating element enclosed
in `[=' and `=]' is an equivalence class, standing for the
sequences of characters of all collating elements equiva-
lent to that one, including itself. (If there are no
other equivalent collating elements, the treatment is as
if the enclosing delimiters were `[.' and `.]'.) For
example, if o and o^ are the members of an equivalence
class, then `[[=o=]]', `[[=o^=]]', and `[oo^]' are all syn-
onymous. An equivalence class may not|- be an endpoint of
4.4BSD June 4, 1993 2
RE_FORMAT(7) BSD Reference Manual RE_FORMAT(7)
a range.
Within a bracket expression, the name of a _c_h_a_r_a_c_t_e_r _c_l_a_s_s
enclosed in `[:' and `:]' stands for the list of all char-
acters belonging to that class. Standard character class
names are:
alnum digit punct
alpha graph space
blank lower upper
cntrl print xdigit
These stand for the character classes defined in _c_t_y_p_e(3).
A locale may provide others. A character class may not be
used as an endpoint of a range.
There are two special cases|- of bracket expressions: the
bracket expressions `[[:<:]]' and `[[:>:]]' match the null
string at the beginning and end of a word respectively. A
word is defined as a sequence of _a_l_n_u_m characters (as
defined by _c_t_y_p_e(3)) which is neither preceded nor fol-
lowed by _a_l_n_u_m characters. This is an extension, compati-
ble with but not specified by POSIX 1003.2, and should be
used with caution in software intended to be portable to
other systems.
In the event that an RE could match more than one sub-
string of a given string, the RE matches the one starting
earliest in the string. If the RE could match more than
one substring starting at that point, it matches the
longest. Subexpressions also match the longest possible
substrings, subject to the constraint that the whole match
be as long as possible, with subexpressions starting ear-
lier in the RE taking priority over ones starting later.
Note that higher-level subexpressions thus take priority
over their lower-level component subexpressions.
Match lengths are measured in characters, not collating
elements. A null string is considered longer than no
match at all. For example, `bb*' matches the three middle
characters of `abbbc', `(wee|week)(knights|nights)'
matches all ten characters of `weeknights', when `(.*).*'
is matched against `abc' the parenthesized subexpression
matches all three characters, and when `(a*)*' is matched
against `bc' both the whole RE and the parenthesized
subexpression match the null string.
If case-independent matching is specified, the effect is
much as if all case distinctions had vanished from the
alphabet. When an alphabetic that exists in multiple
cases appears as an ordinary character outside a bracket
4.4BSD June 4, 1993 3
RE_FORMAT(7) BSD Reference Manual RE_FORMAT(7)
expression, it is effectively transformed into a bracket
expression containing both cases, e.g. `x' becomes `[xX]'.
When it appears inside a bracket expression, all case
counterparts of it are added to the bracket expression, so
that (e.g.) `[x]' becomes `[xX]' and `[^x]' becomes
`[^xX]'.
No particular limit is imposed on the length of REs|-.
Programs intended to be portable should not employ REs
longer than 256 bytes, as an implementation can refuse to
accept such REs and remain POSIX-compliant.
Obsolete (``basic'') regular expressions differ in several
respects. `|', `+', and `?' are ordinary characters and
there is no equivalent for their functionality. The
delimiters for bounds are `\{' and `\}', with `{' and `}'
by themselves ordinary characters. The parentheses for
nested subexpressions are `\(' and `\)', with `(' and `)'
by themselves ordinary characters. `^' is an ordinary
character except at the beginning of the RE or|- the begin-
ning of a parenthesized subexpression, `$' is an ordinary
character except at the end of the RE or|- the end of a
parenthesized subexpression, and `*' is an ordinary char-
acter if it appears at the beginning of the RE or the
beginning of a parenthesized subexpression (after a possi-
ble leading `^'). Finally, there is one new type of atom,
a _b_a_c_k _r_e_f_e_r_e_n_c_e: `\' followed by a non-zero decimal digit
_d matches the same sequence of characters matched by the
_dth parenthesized subexpression (numbering subexpressions
by the positions of their opening parentheses, left to
right), so that (e.g.) `\([bc]\)\1' matches `bb' or `cc'
but not `bc'.
SSEEEE AALLSSOO
regex(3)
POSIX 1003.2, section 2.8 (Regular Expression Notation).
BBUUGGSS
Having two kinds of REs is a botch.
The current 1003.2 spec says that `)' is an ordinary char-
acter in the absence of an unmatched `('; this was an
unintentional result of a wording error, and change is
likely. Avoid relying on it.
Back references are a dreadful botch, posing major prob-
lems for efficient implementations. They are also some-
what vaguely defined (does `a\(\(b\)*\2\)*d' match
`abbbd'?). Avoid using them.
4.4BSD June 4, 1993 4
RE_FORMAT(7) BSD Reference Manual RE_FORMAT(7)
1003.2's specification of case-independent matching is
vague. The ``one case implies all cases'' definition
given above is current consensus among implementors as to
the right interpretation.
4.4BSD June 4, 1993 5