The vector looks like that:

c("Dave_AC11_2_25po_1_1_297", "Dave_AC44_2_25po_1_1_295", "Dave_AC10_2_25po_295", 
  "Dave_AC2_2_25po_1_1_291_por", "Dave_AC42_2_25po_1_1_295", "Dave_AC10_2_25po_1_1_211", 
  "Dave_AC1_2_25po_1_1_295", "Dave_AC32_2_25po_1_35", "Dave_AC38_2_25po_1_15", 
  "Dave_AC12_2_25po_1_1_233")

I am interested only in the part ACX. The vector is million of strings and that’s only example. The problem is that string may have different length and after AC it can be a number from 1 to 99, so one or two characters. Is there any way to extract only AC with a number after that from whole vector ?