BBP_GET_STATE_MONITOR alert messages

In calling FM

CALL FUNCTION 'BBP_GET_STATE_MONITOR'

    EXPORTING

      monitor        = gs_monitor-lv_monitor

      monitorset     = gs_monitor-lv_monitorset

    IMPORTING

      actualvalue    = lv_actualvalue

      alert_message  = lv_alertmsg

      return1        = lv_return

    TABLES

      attribut_table = gt_attrtable.

I will get alert_message(PO having errors-error messages).For one PO

lv_alertmsg  =  Purchase order 4200000904: Purchase order still contains faulty items,

for another  PO

lv_alertmsg  =  PO 4200000906(PR:12345) Purchase order still contains faulty items,So like this for another  PO alertmsg may be different

From lv_alertmsg i need only PO number(4200000904,4200000904.How to get it.