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regular expressions resources and regular expression problem

Asked by 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

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Answer by Andrei Bobrov on 30 Sep 2012
Edited by 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')
```

Patrick Mboma on 30 Sep 2012

Hi Andrei,

Thanks for the reply. My sense is that the patterns are not robust enough. The variables must start with either 'a', 'd', or 'ee' and nothing else. For instance, 'f_1' would not be a valid variable name. As for the functions, how do you detect that a particular expression is a function? would it also capture functions of several arguments? would it capture functions with no arguments?

Thanks alot Pat.

Andrei Bobrov on 30 Sep 2012

Corrected of var

Daniel on 30 Sep 2012

Your regexp foo is so much better than mine. I always have to make things up.

Answer by 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".

Answer by Daniel 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.

Answer by 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.

Walter Roberson on 30 Sep 2012

I think there is an O'Reilly book on regular expressions.

The regular expressions used in MATLAB are a mish-mash between capabilities in other languages. They have similarities to Perl, so it might make sense to study the resources for regular expressions in Perl (and there is probably a book or three for that topic alone.)

Patrick Mboma on 30 Sep 2012

Thanks Walter. It is something I will look into.