Replace space in a string to underscore

Hello All,

I would like to replace space in a string to an underscore.

Eg : 'LN MAT' should be changed to LN_MAT. 

I tried this code

data : ls_text type mara-matnr.

REPLACE ALLOCCURRENCES OF SPACES

       of ls_text with '_".

Runtime exception REPLACE_INFINITE_LOOP  is thrown. Could anyone please help

me out as to what is the solution.

Thanks

Sudha