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Re: Regex with condition

by AnomalousMonk (Bishop)
on Nov 18, 2015 at 23:55 UTC ( #1148069=note: print w/replies, xml ) Need Help??


in reply to Regex with condition

If

  • you want to substitute  'Weis' with  'Schwarz' (I will use only latin characters — simpler for me) only in those lines that immediately follow a line with a  'b' in it, and
  • (update: a  'b' never occurs in a line that has a  'Weis' you want to change, and)
  • you can slurp the entire file into memory at once, and
  • your Perl version is 5.10 or greater (for the  \K operator),
here's a way:
c:\@Work\Perl\monks>perl -wMstrict -le "use 5.010; ;; my $t = qq{ .....P1p1/6P1/7P/5PK1/8 w - - 4 34]\n} . qq{ {Weis am Zug} * .....\n} . qq{1n5P/1P4PK/1q6 b - - 2 42]\n} . qq{ {Weis am Zug} * .....\n} . qq{.......5K2/3R4 w - - 1 33]\n} . qq{ {Weis am Zug} *\n} ; print qq{[[$t]]}; ;; my $pre = qr{ \S+ \s }xms; my $post = qr{ \s [^\n]+ \n [^\{]+ \{ }xms; ;; $t =~ s{ ^ $pre b $post \K Weis \b} {Schwarz}xmsg; print qq{[[$t]]}; " [[ .....P1p1/6P1/7P/5PK1/8 w - - 4 34] {Weis am Zug} * ..... 1n5P/1P4PK/1q6 b - - 2 42] {Weis am Zug} * ..... .......5K2/3R4 w - - 1 33] {Weis am Zug} * ]] [[ .....P1p1/6P1/7P/5PK1/8 w - - 4 34] {Weis am Zug} * ..... 1n5P/1P4PK/1q6 b - - 2 42] {Schwarz am Zug} * ..... .......5K2/3R4 w - - 1 33] {Weis am Zug} * ]]
(Also note that I left out all the  " (double-quote) characters — they confuse my REPL.) If your Perl version is pre-5.10, let me know; there's a simple work-around.

Update: If your file is too large to fit entirely in memory and you must process line-by-line in a while-loop, your code might look something like this (untested):

use 5.010; ... my $pre_b = qr{ ... }xms; my $pre_weis = qr{ ... }xms; my $b_was_seen; while (my $line = <$input_filehandle>) { # check for trigger condition/substitution pattern sequence. # ASSUME: 'b' and 'Weis' cannot both occur in same line. if ($line =~ m{ \A $pre_b b }xms) { # flag 'b' was seen in this line. $b_was_seen = 1; } elsif ($b_was_seen) { # attempt substitution if 'b' seen in previous line. $line =~ s{ \A $pre_weis \K Weis }{Schwarz}xms; $b_was_seen = 0; } # write each (possibly altered) line to output file. print $output_filehandle $line; }
(Again, this assumes your Perl version is 5.10+.) The if-test could also be done, perhaps more concisely, with a  .. flip-flop operator (see Range Operators in scalar context in perlop), but verbosity may be a virtue here.


Give a man a fish:  <%-{-{-{-<

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