Replace all Numeric Values with Single Character
Based on your latest comment, I made few changes to your original example.  The following logic prevents repetitive % from being stuffed.  

See if this works for you.

DECLARE @Temp NVARCHAR(150) = 'Numara_03_09_2018_07_41_04_OP_NIP_CSA_5739038.XLS'
DECLARE @KeepValues AS VARCHAR(50)

SET @KeepValues = '%[^a-z_%. ]%' -- keep underscore and space

WHILE PatIndex(@KeepValues, @Temp) &gt; 0
BEGIN
    SET @Temp = CASE 
            WHEN substring(@temp, PatIndex(@KeepValues, @Temp) - 1, 1) = '%'
                THEN Stuff(@Temp, PatIndex(@KeepValues, @Temp), 1, '')
            ELSE Stuff(@Temp, PatIndex(@KeepValues, @Temp), 1, '%')
            END
END

SELECT @Temp




Result

Numara_%_%_%_%_%_%_OP_NIP_CSA_%.XLS