How to extract port number in listener.ora and store it in a variable

the content of listener.ora file is as follow

# listener.ora Network Configuration File: C:\app\thomas\product\11.2.0\dbhome_1\NETWORK\ADMI
N\listener.ora
# Generated by Oracle configuration tools.

#Default LISTENER.ORA file

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_DBNAME = THOMAS)
      (SID_NAME = THOMAS)
    )
    (SID_DESC =
      (GLOBAL_DBNAME = Oracle8)
      (SID_NAME = ORCL)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = bn-thomas)(PORT = 1521))
    )
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1))
    )
  )

ADR_BASE_LISTENER = C:\app\thomas

how do I extract the port number 1521 and store it in a variable called EXISTING_PORT?

thanks a lot!