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in reply to creating qr from existing regex

Remove the slashes, extract the modifier ("i") (say, in a variable $flags). Keep the core of the regex as a string, in your example that would be .*uba$.

Then put "(?$flags:" in front of it, and ")" behind it. You can use that string ($re) directly as a regex: 

$re = "(?:$flags:$core)";
if($input =~ $re) { ... }

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edit Oops, one too many colons: 
$re = "(?$flags:$core)";
if($input =~ $re) { ... }

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You can also turn it into a regex object: 
$qr = qr/$re/;

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Re^2: creating qr from existing regex
by AnomalousMonk (Archbishop) on Feb 09, 2018 at 21:07 UTC
    $re = "(?:$flags:$core)";

    That doesn't work: too many  ':' (colons): 

    c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*uba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($core, $flags) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $re = qq{(?:$flags:$core)};
 print $re;
 print 'A: match' if 'uba' =~ $re;
"
(?:i:.*uba$)

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    Something like this works: 
    c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/i';
 ;;
 my $flag = qr{ [msixpodual] }xms;
 ;;
 my $convertable =
 my ($pattern, $modifiers) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $rx = qr{(?$modifiers)$pattern};
 print $rx;
 print 'A: match' if 'u/Ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry
 ;;
 print 'B: match' if 'foolubatU/bA' =~ $ry;
 print 'C: match' if 'FoolubatU/bA' =~ $ry;
"
(?^:(?i).*u\/ba$)
A: match
(?^msx: \A foo (?^:(?i).*u\/ba$) )
B: match

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    Note that:
    • I go directly to  qr// form.
    • The modifiers ($flag pattern) implemented vary by Perl version; see perlop.

    Give a man a fish:  <%-{-{-{-<

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[d/l]
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      Yes, "(?:$flags:$pattern)" is wrong, it should have been "(?$flags:$pattern)". That is: the flags between the leading "(?" and ":" of the standard non-capturing grouping parens, i.e. "(?:$pattern)".

      You can always try to print out a qr/foo/i, to see what perl does natively.

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