replacing a number in a string

Hi i have a string s='21-x1 4000. 4000. 0.'and i would like to replace the first 4000 without replacing the second 4000, i did use

strrep(s,'4000','1512.45')

but the outcome was ns = '21-x1 1512.45 1512.45 0.'

thanks,

Answer 1

[Walter Roberson]

s='21-x1 4000. 4000. 0.';
regexprep(s, '4000', '1512.45', 'once')

[Abdulrahman Metawa]

many thanks