(?i-msx:\bBug {0,1}(\d+):)

matches as follows:
  
NODE                     EXPLANATION
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(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
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  \b                       the boundary between a word char (\w) and
                           something that is not a word char
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  Bug                      'Bug'
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   {0,1}                   ' ' (between 0 and 1 times (matching the
                           most amount possible))
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  (                        group and capture to \1:
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    \d+                      digits (0-9) (1 or more times (matching
                             the most amount possible))
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  )                        end of \1
----------------------------------------------------------------------
  :                        ':'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------