# regular expressions resources and regular expression problem

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Patrick Mboma on 30 Sep 2012
Hi all, I don't know anything about regular expressions and I have two questions:
1. What is the most effective way to learn regular expressions and where to find the best possible tutorials, examples, exercises etc.
2. I have the following problem. I would like to extract (1) a list variables and (2) a list of matlab functions from a string. The variables have a precise definition: (i) they can start with 'a' 'd' or 'ee'; (2) then an underscore '_'; (3) then an integer 'n'. For example, the variables could be 'a_1', 'd_100', 'ee_3289' and the expression to parse could look like: 'cos(exp(a_2/4*d_1^3-ee_4)+d_85)'. In this example, the list of variables is 'a_2', 'd_1', 'ee_4', 'd_85' and the list of matlab functions is 'cos', 'exp'. My question here is what is the (best) regular expression to extract that information?
Thanks, Pat

Andrei Bobrov on 30 Sep 2012
Edited: Andrei Bobrov on 30 Sep 2012
2.
str = 'cos(exp(a_2/4*d_1^3-ee_4)+d_85)';
fun = regexp(str,'\w*(?=\()','match')
var = regexp(str,'(a|d|ee)_\d*','match')
Daniel Shub on 30 Sep 2012
Your regexp foo is so much better than mine. I always have to make things up.

Ned Gulley on 8 May 2013
Another way to get good at regular expressions is to practice on Cody with all the problems that have been tagged "regexp".

Daniel Shub on 30 Sep 2012
Assuming a string x,
[a, b] = regexp(x, '(a|d|(ee))(_)([0-9]*)', 'start', 'end')
will find the start and end indices of your "variables." This will allow for a variable a_0 which may or may not be desired. Without a rule for defining functions that handles the numerous edge cases, creating the regexp is difficult. Andrei's answer seems reasonable to me.

Patrick Mboma on 30 Sep 2012
Thank you both. That is one big problem solved. The biggest remains though: how do I become as good as you guys are with regular expressions?
Thanks a lot, Pat.
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Patrick Mboma on 30 Sep 2012
Thanks Walter. It is something I will look into.