vb.net - Clear rich text box and leave certain strings -

i have lots of data in rich text box, trying clear , leave words begin az, possible @ all? have tried following coding fails

string cannot of 0 length. parameter name: oldvalue 

the code is

private sub button3_click(sender system.object, e system.eventargs) handles button3.click         dim str string         str = rtbjw.text         if not str.contains("az") = true             rtbjw.text = rtbjw.text.replace("", "").trim()         end if     end sub 

example text

az0034   14         az0034l1    art3    ltd   0 srv01   000_00000_ltd   ecr   0  -   active    - az0003   10         az0003l1    art3    ltd   0 srv01   000_00000_ltd   ecr   8  -   active    - az0008    9         az0008l1    art3    ltd   0 srv01   000_00000_ltd   ecr  12  -   active    - az0010    8         az0010l1    art3    ltd   0 srv01   000_00000_ltd   ecr   7  -   active    - az0009    7         az0009l1    art3    ltd   0 srv01   000_00000_ltd   ecr   7  -   active    - az0004    2         az0004l1    art3    ltd   0 srv01   000_00000_ltd   ecr   3  -   active    - az0006    1         az0006l1    art3    none      - -   --- ecr   7  -   active    - az0007    -         az0007l1    art3    none      - -   jo  ecr   8  -   active    - 

rtbjw.text = rtbjw.text.replace("", "").trim()

will replace empty strings found within rtjbw.text empty strings. i'm sure noticed didn't anything, because there aren't empty strings replace. @ rate, if want leave words begin in "az" not right approach. few different methods, including regex work. however, i'm not familiar regex enough provide solution using it, work well.

private sub button3_click(sender object, e eventargs) handles button3.click     dim strarr() string = rtbjw.text.split(" ")     rtbjw.clear()     each item in strarr         if item.startswith("az")             if rtbjw.text.length > 0 rtbjw.text &= " "             rtbjw.text &= item         end if     next end sub 

what did here take contents of rtbjw , assign string array, splitting space. creates list of "words" (assuming each word separated space). since have list of "words" in textbox, textbox cleared of text.

then, in loop, i'm looking @ each item in list of words, , if begins "az" (using startswith() method), i'm doing 2 things. first, if length of text in textbox greater 0, add space what's in textbox (to separate words, , greater 0 check prevent adding space first character), adding word begins "az" end of text in textbox.

edit: because curious , i'm learning how use them, modified use linq query instead. same thing, instead of assigning text string array, text stored in queryresult variable. see following.

private sub button3_click(sender object, e eventargs) handles button3.click     dim queryresults ienumerable = word in rtbjw.text.split(" ")                                       word.startswith("az")     rtbjw.clear()     each word in queryresults         if rtbjw.textlength > 0 rtbjw.text &= " "         rtbjw.text &= word     next end sub