Re: regex anchoring issue

[kcott]

in reply to regex anchoring issue

G'day penguin-attack,

Welcome to the monastery.

Firstly, your data description seems a little ambiguous: you say "end character" then describe <SOH> (5 chars), ^A (2 chars) and Ctrl-A (1 char). If, by <SOH>, you mean the ASCII character - that is the same character as Ctrl-A (i.e. the character with the ASCII value of 1).

Your main problem in your regexp is the use of a character class (i.e. [...]) - see Character Classes and other Special Escapes under perlre - Regular Expressions for details. You also don't need the 'g' modifier in either the match (m/.../) or the split function.

The following script does what I think you want (in terms of identifying the line endings). If not, please provide some sample data with expected output to remove the ambiguity I mentioned at the start.

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my $soh_string = 'soh_string<SOH>';
my $caret_a_string = 'caret_a_string^A';
my $ctrl_a_string = 'ctrl_a_string' . chr(1);

my $test_string = join('',
    $soh_string, $caret_a_string, $ctrl_a_string,
    $caret_a_string, $ctrl_a_string, $soh_string,
    $ctrl_a_string, $soh_string, $caret_a_string
);

my $string_re = qr{(?><SOH>|\^A|\cA)};

say for split $string_re => $test_string;

Output:

$ pm_soh_split.pl
soh_string
caret_a_string
ctrl_a_string
caret_a_string
ctrl_a_string
soh_string
ctrl_a_string
soh_string
caret_a_string

[smls]

Why do you place the regex in a (?>...) non-backtracking group?

[kcott]

Wrapping regexp alternations in (?>...) is something I do by default. While there may be rare cases where this might be problematical, I haven't encountered any: it's something that doesn't hurt and, indeed, often helps.

This usage is based on a "Perl Best Practices" guideline: Backtracking (page 269). It's summarised on page 271 as:

... rewrite any instance of:

X | Y

as:

(?> X | Y )

While I'm not a slave to all "Perl Best Practices" guidelines, this is one I have found to be useful.

Update: s/have encountered/haven't encountered/

[BillKSmith]

Refer to charnames for a neat way to code the value of your $soh_string.

use charnames qw(:full);
$soh_string = "\N{SOH}";

[kcott]

Thanks, Bill. I had considered that but decided not to use it due to the ambiguity I noted in my opening paragraph. Had penguin-attack wanted the single ASCII character SOH, instead of the string '<SOH>', that was covered by Ctrl-A (also noted).

[Side issue (struggling not to appear grossly pedantic): the charnames pragma has been distributed with Perl since at least v5.8.8 - the perldoc link (charnames) would provide the most recent documentation.]