I'm trying to search a file for several matching blocks of text. A sample
of what I'm searching through is below.
What I want to do is match "##### START block #####" through to the next
"##### END block #####" and repeat that throughout the file without
matching any of the text that falls between each matched block (that is,
the "ok: some text" lines should not be matched). Here is the one-liner I'm
using:
I've tried a few variations of this but with the same result--a match is
being made from the first "##### START block #####" to the last "##### END
block #####", and everything in between... I believe that the ".*",
combined with the "s" modifier, in the regex is causing this match to be
made.
What I'm not sure how to do is tell Perl to search from START to the next
END and then start the search pattern over again with the next START-END
match.
On Thu, Nov 01, 2012 at 12:44:08AM -0700, Thomas Smith wrote:
> Hi,
> I'm trying to search a file for several matching blocks of text. A sample
> of what I'm searching through is below.
> What I want to do is match "##### START block #####" through to the next
> "##### END block #####" and repeat that throughout the file without
> matching any of the text that falls between each matched block (that is,
> the "ok: some text" lines should not be matched). Here is the one-liner I'm
> using:
> I've tried a few variations of this but with the same result--a match is
> being made from the first "##### START block #####" to the last "##### END
> block #####", and everything in between... I believe that the ".*",
> combined with the "s" modifier, in the regex is causing this match to be
> made.
> What I'm not sure how to do is tell Perl to search from START to the next
> END and then start the search pattern over again with the next START-END
> match.
> How might I go about achieving this?
perl -ne 'print if /##### START block #####/ .. /##### END block #####/' file.txt
> I'm trying to search a file for several matching blocks of text. A sample
> of what I'm searching through is below.
> What I want to do is match "##### START block #####" through to the next
> "##### END block #####" and repeat that throughout the file without
> matching any of the text that falls between each matched block (that is,
> the "ok: some text" lines should not be matched). Here is the one-liner I'm
> using:
> I've tried a few variations of this but with the same result--a match is
> being made from the first "##### START block #####" to the last "##### END
> block #####", and everything in between... I believe that the ".*",
> combined with the "s" modifier, in the regex is causing this match to be
> made.
The '*' is what's called a "greedy" quantifier. That means it will match as many characters in the string as possible. What the regular expression engine does when it encounters the pattern '.*' is to immediately match it with as many characters as possible. Since your regular expression includes the 's' modifier, this will include newlines as well. When the RE engine sees that there are characters in the pattern after the '.*', it will start removing characters from the end of the substring matched by the '.*' until the subsequent pattern characters are also matched. This will continue until there are no characters matched by the '.*'.
The result of all this is that for your pattern, the last '##### END block #####' substring is the one that will be matched, and the '.*' pattern will match everything between the first '##### START block #####' and the last '##### END block #####'.
The way to fix this is to make the '*' quantifier "non-greedy" by putting a '?' quantifier after it. With that pattern, the RE engine will match as few characters as possible, and the first START block will pair up with the first subsequent END block. A 'g' modifier will tell the RE engine to start looking after each match for the next match in the string.
