> I feel like I am missing something very basic here, but I just can't see it. TIA for any suggestions.

You are getting bitten by what is called "slasheritis" in the emacs-lisp world.

You have to escape backslashes from two interpretations which turns many elisp regexes into an unreadable mess filled with quadruple backslashes \\\\

This works:

#!/usr/bin/perl
use strict;
use warnings;
use Test::More;

my ($text, $search);


$text = "\\x1\\x2\\x3\\x4";
$text =~ s/x2\\.+\\x4/new/;
my $exp = $text;


$text   = "\\x1\\x2\\x3\\x4";
$search = "x2\\\\.+\\\\x4";
$text =~ s/$search/new/;
my $got = $text;


is( $got, $exp, "slasheritis");

done_testing;
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ok 1 - slasheritis
1..1
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there are more elegant solutions in Perl°, but I wanted to clarify the problem.

HTH! :)

°) alas not in GnuEmacs. XEmacs had "raw strings", but became abandon-ware.

°) alas not in GnuEmacs. XEmacs had "raw strings", but became abandon-ware.

While it does not provide raw strings, I've always found the rx macro to offer a very neat solution for building complicated regular expressions, somewhat akin to Perl's /x flag:

(rx "\\x2"
    (one-or-more nonl)
    "\\x4")
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that's true, but you still need two \\ to quote a meta.

here a bad but not exceptional line taken from cperl-mode

"\\((\\(\\\\.\\|[^\\\\()]\\|([^\\\\()]*)\\)*)\\)?"

edit

and more things need escaping in elisp regexes

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

Thanks LanX. You're right. Once I doubled the "double" backslashes it worked fine.

"It's not how hard you work, it's how much you get done."

> Note: I tried running the search string variable through the "qr" function, but that did not help.

qr helps

#!/usr/bin/perl
use strict;
use warnings;

my ($text, $search);

$text = "\\x1\\x2\\x3\\x4";
$search = qr/x2\\.+\\x4/;

print "\nUsing variable in regex: (qr)\n";
print "Before: text = $text\n";
$text =~ s/$search/new/;
print " After: text = $text\n";

__END__

Using variable in regex: (qr)
Before: text = \x1\x2\x3\x4
 After: text = \x1\new
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L*

Thanks Discipulus. I now see the reason my qr didn't work. It was because I first assigned the search string to the variable, then reassigned the variable (run through qr) to itself, which did not double the backslashes.

my $search = "x2\\.+\\x4";
$search = qr($search);
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"It's not how hard you work, it's how much you get done."

You seem to understand this already, but I thought I'd try to make it clear for future readers of this thread: Why doesn't it work?

When you finish slashing away at the $search string, what have you got?

Win8 Strawberry 5.8.9.5 (32)  Sat 05/01/2021 17:03:44
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -l

my $search = "x2\\.+\\x4";
print $search;

^Z
x2\.+\x4
  ^^
  ||
  ++----- backslash escaped literal period (one or more)
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Similar result if you pass the $search string through the qr// regex object constructor.

Win8 Strawberry 5.8.9.5 (32)  Sat 05/01/2021 17:14:10
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -l

my $search = "x2\\.+\\x4";
my $rx_search = qr/$search/;
print $rx_search;

^Z
(?-xism:x2\.+\x4)
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Update: Minor re-formatting for clarity/coherence.

I think the easiest way to put it, is that backslash is the escape character for literal strings as well as for regex meta-characters.

This means they need to be doubled for each step.

Even in single quoted strings, since they allow escaping the delimiter.

AFAIK are here-docs the only way to avoid the need of self-escaping a \

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

AFAIK are here-docs the only way to avoid the need of self-escaping a \

Actually, I was not aware of this, but yeah:

Win8 Strawberry 5.8.9.5 (32)  Sun 05/02/2021 21:06:46
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

my $s = <<'EOT';
\b\\o\\\f\\\\f\
EOT
print "'$s' \n";

$s = q/\q\\u\\\u\\\\x\
/;
print "'$s' \n";

^Z
'\b\\o\\\f\\\\f\
'
'\q\u\\u\\x\
'
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Learned something today! :)

---

Give a man a fish:  <%-{-{-{-<