I have used modifier in compress function and it has solved the problem.
List of Modifiers:
a – Compress or Delete all upper and lower case characters from String.
ak- Compress or Delete alphabetic characters(1,2,3 etc) from String.
kd- Compress or Delete characters(aplabets) from String.( Keeps only digits).
d – Compress or Delete numerical values from String.
i – Compress or Delete specified characters both upper and lower case from String.
k – keeps the specified characters in the string instead of removing them.
l – Compress or Delete lowercase characters from String.
p – Compress or Delete Punctuation characters from String.
s – Compress or delete spaces from String. This is default.
u – Compress or Delete uppercase characters from String.
Hope this helps!