c# - How to allow editing number on a filled masked text box? -

i have masked textbox , want edit number on middle of textbox changing number of right number typed.

for example:

111.511-1/11

i want change number 5 9. i'll type de left arrow until near dot. when near dot , type 9, number 5 disappear , number 9 appear.

111.911-1/11

i thought need capture left arrow typing changing. dont know how capture it.

any code suggestions?

xaml

     <textbox textchanged="formataprocesso" keydown="numberonly"/> 

c#

     private void formataprocesso(object sender, eventargs e)         {             regex regex1 = new regex("[0-9]{4}$");             regex regex2 = new regex("[0-9]{3}.[0-9]{4}$");                 regex regex3 = new regex("[0-9]{3}.[0-9]{3}-[0-9]{2}$");     }        if (regex3.ismatch(process.text))                 {                     isvalortratado = true;                     processo.text = string.format("{0}/{1}", process.text.substring(0, 9), process.text.substring(9, 1));                      process.selectionstart = process.text.length;                     process.selectionlength = 0;                 }                 else if (regex2.ismatch(process.text))                 {                      processo.text = string.format("{0}-{1}", process.text.substring(0, 7), process.text.substring(7, 1));                      process.selectionstart = process.text.length;                     process.selectionlength = 0;                 }   .... 

thanks corrections, guys.

i used it:

            int position = numprocess.selectionstart;              if (numprocess.text.length <= position)             {                ...             }             else             {                 string firstpiece = numprocess.text.substring(0, position);                 string secondpiece = numprocess.text.substring(position + 1);                  numprocess.text = firstpiece + secondpiece ;                  numprocess.selectionstart = position;                 numprocess.selectionlength = 0;             } 

wiki