Basically the same as Rohits last post, but without need to specify the leading part of the source strings:

s1 = "aaaa bbb (xyz / cc.dd ee) : = 3.456e2";
s2 = "pqr : = 345.6";
s3 = "stu / (v w x) : = 345";

Last /@ StringSplit[{s1, s2, s3}]
(* {"3.456e2","345.6","345"} *)

Appreciate your help. I went through the post and the other one mentioned there. Not understood all the steps yet, but my problem is more of copying a numerical value after a string in the text file, rather than changing its accuracy or format. I am bit confused now, whether I need to convert the string to CForm for Mathematica to read it in the text file. However, my previous experience is that Mathematica can read a string in a text file, as it is. Please correct me if I am wrong.

In my case, it would be fine if I am able to copy a number in its given form, as long as it is of correct numerical value for all possible formats. For example, if the number is 1.23e2 in the text file, I am not worried whether I get the output as 1.23e2 or 123 or 123.00 or 1.23*10^2 etc.

Hope it explains my problem little better. Thanks once again.

Thanks. It worked for any kind of numerical value. However, if the i/p has more than one strings present, this method does not work. For example if

s4="pqr : = 3
 aaaa bbb (xyz / cc.dd ee) : = 3456e2";
extract /@ s4

error message is -

Read::readn: Invalid real number found when reading from StringToStream[pqr : = 5].

This point is important as I have to use this method to extract a parameter from a text file having large number of strings, each followed by a number.

Thanks for your help once again..

Well, now you have changed the requirements. It would have been better if your original question stated the requirements clearly and completely.

Actually, I mentioned that I need to copy a number from the text file, after a particular string. Anyways, finally, StringSplit worked, however I had to make some changes to what you suggested. Thanks for the help.