The following are the valid
symbols and associated meanings:
. A period (.) is a one-character RE that matches any
character except new-line.
* 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.
^ A circumflex (^) at the beginning of an entire RE con-
strains that RE to match an initial segment of a line.
$ A dollar sign ($) at the end of an entire RE con-
strains that RE to match a final segment of a line.
[] A non-empty string of characters enclosed in square
brackets ([]) is a one-character RE that matches any
one character in that string. If, however, the first
character of the string is a circumflex (^), the one-
character RE matches any character except new-line and
the remaining characters in the string. The ^ has this
special meaning only if it occurs first in the string.
The minus (-) may be used to indicate a range of con-
secutive characters; for example, [0-9] is equivalent
to [0123456789]. The - loses this special meaning if
it occurs first (after an initial ^, if any) or last
in the string. The right square bracket (]) does not
terminate such a string when it is the first character
within it (after an initial ^, if any); for example,
[]a-f] matches either a right square bracket (]) or
one of the ASCII letters a through f inclusive.
+ A regular expression followed by + means one or more
times. For example, [0-9]+ is equivalent to
[0-9][0-9]*.
{m} {m,} {m,u}
Integer values enclosed in {} indicate the number of
times the preceding regular expression is to be
applied. The value m is the minimum number and u is a
number, less than 256, which is the maximum. If only m
is present (that is, {m}), it indicates the exact
number of times the regular expression is to be
applied. The value {m,} is analogous to {m,infinity}.
The plus ( +) and star (*) operations are equivalent
to {1,} and {0,} respectively.
( ... )$n
n can have value from 0 to 4. The value of the
enclosed regular expression is stored and can be used
with variable $0, $1, $2, $3, $4. At most, five
enclosed regular expressions are allowed.
( ... )
For compatibility with previous version ( ... ) is
considered as ( ... )$1
