Side effects of an /e modifier

timpoiko has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks If I have a function foo(),
sub foo {my $c = shift; $c*=5; return $c;}
All those substitutions works as expected:

$bar =~ s/(\d{2})/joo/msg;
$bar =~ s#(\d{2})#jee#msg;
$bar =~ s#(\d{2})#foo($1)#mesg;
$bar =~ s#(\d{2})#<td>ha ha ha</td>#msg;
[download]

But if want to use all things: a /e modifier, html tags and # instead of / (because \-escaped slashes are not very readable)
$bar =~ s#(\d{2})#<td>foo($1)</td>#mesg;
I only get

Bareword found where operator expected at a.pl line 18, near "<td>foo"
        (Missing operator before foo?)
syntax error at a.pl line 18, near "<td>foo"
Search pattern not terminated at a.pl line 18.
[download]

man perlop is not very helpful.

Any non-whitespace delimiter may replace the slashes. Add space after the "s" when using a character allowed in identifiers. If single quotes are used, no interpretation is done on the replacement string (the "/e" modifier overrides this, however). Note that Perl treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, for example, s(foo)(bar) or s<foo>/bar/. A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second "e" modifier will cause the replacement portion to be "eval"ed before being run as a Perl expression.

Tested in Debian Jessie (5.20) and in Debian Buster (5.29).

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Re: Side effects of an /e modifier by Corion (Patriarch) on Jan 29, 2020 at 14:13 UTC
I think the right hand side in an `s///e` block must be valid Perl. `<td>foo($1)</td>` is not valid Perl. Quoting the HTML tags should work: `$bar =~ s#(\d{2})#"<td>".foo($1)."</td>"#mesg;` [download] More code: `#!perl -w sub foo { "<$_[0]>" }; my $bar = '2299-33-ff'; $bar =~ s#(\d{2})#"<td>".foo($1)."</td>"#mesg; print $bar; __END__ <td><22></td><td><99></td>-<td><33></td>-ff` [download]	[reply] [d/l] [select]
Re: Side effects of an /e modifier by hippo (Archbishop) on Jan 29, 2020 at 14:11 UTC
`<td>foo($1)</td>` is not a valid expression in Perl. Maybe try `'<td>' . foo($1) . '</td>'` instead?	[reply] [d/l] [select]
Re: Side effects of an /e modifier by k-mx (Scribe) on Jan 29, 2020 at 17:17 UTC
Yet another solution (: You can eval random code inside (?{ }) and use result with variable $^R `$bar =~ s#\d{2}(?{ $& * 5})#<td>$^R</td>#gms;` More information: `perldoc -v '$^R' perldoc -v '$&' perldoc perlre` [download] And I want to point, that you don't need to capture whole match. It will already situated in $& P.S. I should have said, that there is global performance penalty for using $& somewhere in your code. It was fixed only in 5.020 Please read following documentation for more information: `perldoc perlfaq6 perldoc perlvar` [download]	[reply] [d/l] [select]

P.S.