my $input = q'

\cite[page 6]{Dickens}
\cite[page 9]{Dickens}

';

$input =~
s<
    \\cite
    \[
      (
        [^\]]+
       ) # $1
    \]
    {
        (    # $2
         [a-zA-Z]+
         )
    }
>"
\\cite[$1]{\l$2}
"gx;

print $input;
__END__
\cite[page 6]{dickens}


\cite[page 9]{dickens}




use YAPE::Regex::Explain;
die YAPE::Regex::Explain->new(qr<
    \\cite
    \[
      (
        [^\]]+
       ) # $1
    \]
    {
        (    # $2
         [a-zA-Z]+
         )
    }
>x)->explain;
__END__
The regular expression:

(?x-ims:
    \\cite
    \[
      (
        [^\]]+
       )  # $1
    \]
    {
        (     # $2
         [a-zA-Z]+
         )
    }
)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  \\                       '\'
----------------------------------------------------------------------
  cite                     'cite'
----------------------------------------------------------------------
  \[                       '['
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [^\]]+                   any character except: '\]' (1 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \]                       ']'
----------------------------------------------------------------------
  {                        '{'
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    [a-zA-Z]+                any character of: 'a' to 'z', 'A' to 'Z'
                             (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  }                        '}'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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