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* DECLARATIONS                                                        *
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DATA: go_matcher TYPE REF TO cl_abap_matcher.

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* LAYOUT                                                              *
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PARAMETERS: pa_mail TYPE AD_SMTPADR.

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* EVENTS                                                              *
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AT SELECTION-SCREEN.
  IF pa_mail IS NOT INITIAL.
    go_matcher = cl_abap_matcher=>create(
      pattern = '\w+(\.\w+)*@(\w+\.)+(\w{2,4})'
      ignore_case = 'X'
      text = pa_mail ).
    IF go_matcher->match( ) IS INITIAL.
      MESSAGE 'Invalid Mail Address' TYPE 'E'.
    ENDIF.
  ENDIF.