Avoid recursive replacement using regex

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

Hi Monks,
How are you doing today? I am in a little trouble and need your help with regex.
I am replacing some strings in JavaScript. However, I believe the same regex will work for both JavaScript and Perl.

With my method of replacing, I am getting into the problem of recursive replacement.
I want to do the following.
1. Replace & with &
2. Replace &lt; with <
3. Replace &gt; with >
4. Replace < with <
5. Replace > with >

I tried to include the following code snippets in my JS code.

new_json_string = new_json_string.replace(/&amp;/g, '&');
new_json_string = new_json_string.replace(/&lt;/g, '<');
new_json_string = new_json_string.replace(/&gt;/g, '>');
[download]

But the problem with the above code snippet is, both text segments like [<] and [&lt;] will be replaced ultimately with [<] symbol. Is it possible to include all the three replacements in a single statement to avoid recursive replacement?
Your help is highly appreciated, Thank you very much in advance.

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Re: Avoid recursive replacement using regex
by parv (Parson) on Nov 25, 2014 at 11:34 UTC

You have the wrong order of replacements. Do replacement 4|5 first; then 1 (2|3 are obtained for free).

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Re: Avoid recursive replacement using regex
by Athanasius (Archbishop) on Nov 25, 2014 at 12:09 UTC

Hello anirudhkumar_r,

Update: parv’s solution is simpler, therefore better!

To prevent recursive replacement, you can use negative look-behind assertions as follows:

#! perl
use strict;
use warnings;

my $s = '&amp;lt; &lt; &amp;xyz &amp;gt;abc 123&gt;456 me&amp;you';
   $s =~ s/ (?<!&amp;) &lt; /</gx;
   $s =~ s/ (?<!&amp;) &gt; />/gx;
   $s =~ s/     &amp;lt;    /&lt;/gx;
   $s =~ s/     &amp;gt;    /&gt;/gx;
   $s =~ s/     &amp;       /&/gx;

print $s;
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Output:

22:03 >perl 1084_SoPW.pl
&lt; < &xyz &gt;abc 123>456 me&you
22:03 >
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See “Look-Around Assertions” in perlre#Extended-Patterns.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: Avoid recursive replacement using regex
by AnomalousMonk (Archbishop) on Nov 25, 2014 at 14:34 UTC

Another way. This has the advantage of being completely independent of ordering of operations, and more replacement string mappings can easily be added to the %replace hash.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;
+';
 print qq{'$s'};
 ;;
 my %replace = ( amp => '&',  lt  => '<',  gt  => '>', );
 my ($find) =
   map qr{ & ($_) ; }xms,
   join '|',
   keys %replace
   ;
 $s =~ s{$find}{$replace{$1}}xmsg;
 print qq{'$s'};
"
'&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;'
'&  &lt;  &gt;  <  >  &<  &&'
[download]

(Maybe ask on a Java site?)

Update: Here's a variation that has the advantage of avoiding what I think of as 'hidden' capture groups in qr// objects: captures that can confuse group counting when these objects are used to compose larger regexes. In the following, all captures are at the 'top' level in the m// match, so counting's easy.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;
+';
 print qq{'$s'};
 ;;
 my %replace = ( amp => '&',  lt  => '<',  gt  => '>', );
 my ($entity) =
   map qr{ $_ }xms,
   join ' | ',
   keys %replace
   ;
 print $entity;
 ;;
 $s =~ s{ & ($entity) ; }{$replace{$1}}xmsg;
 print qq{'$s'};
"
'&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;'
(?^msx: lt | gt | amp )
'&  &lt;  &gt;  <  >  &<  &&'
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Re: Avoid recursive replacement using regex
by Loops (Curate) on Nov 25, 2014 at 12:39 UTC

Sure, just translate this into JavaScript:

my %k = qw(l < g > & &);
print "$_ to ".s/(&)amp;|&(l|g)t;/$k{$+}/reg.$/
   for qw( &amp; &amp;lt; &amp;gt; &lt; &gt; );
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&amp; to &
&amp;lt; to &lt;
&amp;gt; to &gt;
&lt; to <
&gt; to >
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