06-23-2011
In the pattern .*3 matches all characters up to the character 3. Because sed uses a greedy match, it matches all characters up to the last three.
Adding the parens \(.*\)3 'groups' the part of the input string that was matched by the enclosed pattern. In this case, all characters before the 3.
In the replacement string, the \1 is a 'back reference' to the first group, so what ever was matched by the first group is 'kept' in the output string. The remainder of the substitution string is then added.
Because the text following the last occurrence of the desired character is NOT matched, it is left alone in the replacement process and appears as it was in the input string.
I hope this makes sense.
Last edited by agama; 06-23-2011 at 11:51 PM..
Reason: typo
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REGEXP(6) Games Manual REGEXP(6)
NAME
regexp - regular expression notation
DESCRIPTION
A regular expression specifies a set of strings of characters. A member of this set of strings is said to be matched by the regular
expression. In many applications a delimiter character, commonly bounds a regular expression. In the following specification for regular
expressions the word `character' means any character (rune) but newline.
The syntax for a regular expression e0 is
e3: literal | charclass | '.' | '^' | '$' | '(' e0 ')'
e2: e3
| e2 REP
REP: '*' | '+' | '?'
e1: e2
| e1 e2
e0: e1
| e0 '|' e1
A literal is any non-metacharacter, or a metacharacter (one of .*+?[]()|^$), or the delimiter preceded by
A charclass is a nonempty string s bracketed [s] (or [^s]); it matches any character in (or not in) s. A negated character class never
matches newline. A substring a-b, with a and b in ascending order, stands for the inclusive range of characters between a and b. In s,
the metacharacters an initial and the regular expression delimiter must be preceded by a other metacharacters have no special meaning and
may appear unescaped.
A matches any character.
A matches the beginning of a line; matches the end of the line.
The REP operators match zero or more (*), one or more (+), zero or one (?), instances respectively of the preceding regular expression e2.
A concatenated regular expression, e1e2, matches a match to e1 followed by a match to e2.
An alternative regular expression, e0|e1, matches either a match to e0 or a match to e1.
A match to any part of a regular expression extends as far as possible without preventing a match to the remainder of the regular expres-
sion.
SEE ALSO
awk(1), ed(1), sam(1), sed(1), regexp(2)
REGEXP(6)