Tip #9 from the Basic debugging checklist: YAPE::Regex::Explain 

The regular expression:

(?-imsx:^.T2|S)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  T2                       'T2'
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  S                        'S'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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You should use parentheses to group the alternation: 
/^.T(2|S)/

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Or, if there is no need to capture: 
/^.T(?:2|S)/

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In reply to Re: Perl alternation regex looks ok to me? by toolic
in thread Perl alternation regex looks ok to me? by misterperl

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