06-25-2013
The sed command uses a substitute command ( s/.../.../ ) . The first part of the expression contains a basic regular expression (regex). The escaped parentheses \( and \) are used to group parts of matched text that can be back referenced by \1 in the second part of the s-command.
So in this case only the second colon is outside the grouped part and does not get back referenced, so effectively it gets discarded. If we apply this to your input file then the first match is :8:, which gets substituted with :8|. At the end of the expression is the letter g, which is the "global" flag, which means that the operation should be repeated for every occurrence on the line.
So this will be repeated, and crucial here, is that the next match will start after the previous match, so next up will be :4: which becomes :4| and then :14: becomes :14| and so on...
Last edited by Scrutinizer; 06-25-2013 at 01:49 AM..
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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)