Richard Lee wrote:
John W. Krahn wrote:
Richard Lee wrote:
While reading 'mastering perl', I run into @- and @+ for the first time.
perldoc perlvar
Trying to understand what's going on, I ran the code from the book, but
$-[1] and $+[1] shoudln't match only the first match? (in this case,
shoudln't it be, 7 to 8 ?, instead of 7 to 9 ?)
The length of the first match is 2. 8 - 7 = 1. If $+[1] had the
value 8 then the first match would only have a length of 1.
Look at the @LAST_MATCH_START entry in:
perldoc perlvar
Also read the entries for $<digits>, $MATCH, $PREMATCH, $POSTMATCH,
$LAST_PAREN_MATCH, $^N, @LAST_MATCH_END and $LAST_REGEXP_CODE_RESULT.
And of course the regular expression documentation:
perldoc perlrequick
perldoc perlretut
perldoc perlre
perldoc perlop
Also see:
perldoc -f pos
perldoc -f study
According to the doc,
since $1 is substr($var, $-[1], $+[1] - $-[1] )
and hi matches 7 and 8th elements of the variable and length has to be
2, so
substr($var, 7, 2), but I still don't understand
because if formula was applied directly, it still would be,
substr($var, 7, (8-7)) which would come out to
substr($var, 7, 1).
so, how I am reading it is then, $+[1] is not a value that I read off
the variable but something perl assigns it to it the way it see fit?(in
order to match the first match $1) and since
lenght of the match is 2, $-[1] is fixed(where the match starts) but
$+[1] tries to match the length?
@+ definition does mentions $+[1] is hte offset past where $1 ends.
Say you have the string "abcdefghi".
The positions in the string are:
a b c d e f g h i
^ ^ ^ ^ ^ ^ ^ ^ ^ ^
0 1 2 3 4 5 6 7 8 9
If you have the regular expression:
/(de)/
Then the match starts at position 3, moves forward two characters, and
ends at position 5, where the next match, if any, will start.
If it stopped at position 4 then it would only match 1 character.
If you want to explore all the gory details of regular expressions then
get the book _Mastering Regular Expression_ by Jeffrey E. F. Friedl:
http://www.oreilly.com/catalog/regex3/index.html
John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall
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