Dynamic regexprep in MATLAB

Question

I have the following strings in a long string:

a=b=c=d;
a=b;
a=b=c=d=e=f;

I want to first search for above mentioned pattern (X=Y=...=Z) and then output like the following for each of the above mentioned strings:

a=d;
b=d;
c=d;

a=b;

a=f;
b=f;
c=f;
d=f;
e=f;

In general, I want all the variables to have an equal sign with the last variable on the extreme right of the string. Is there a way I can do it using regexprep in MATLAB. I am able to do it for a fixed length string, but for variable length, I have no idea how to achieve this. Any help is appreciated.

My attempt for the case of two equal signs is as follows:

funstr = regexprep(funstr, '([^;])+\s*=\s*+(\w+)+\s*=\s*([^;])+;', '$1 = $3; \n $2 = $3;\n');

I think you misspelled third case example, shouldn't all be equals to f as you stated? — Rodrigo López, May 01 '15 at 20:22

Hoki · Accepted Answer · 2015-05-01T23:45:36.217

Not a regexp but if you stick to Matlab you can make use of the cellfun function to avoid loop:

str = 'a=b=c=d=e=f;' ; %// input string

list = strsplit(str,'=') ;
strout = cellfun( @(a) [a,'=',list{end}] , list(1:end-1), 'uni', 0).' %'// Horchler simplification of the previous solution below 

%// this does the same than above but more convoluted
%// strout = cellfun( @(a,b) cat(2,a,'=',b) , list(1:end-1) , repmat(list(end),1,length(list)-1) , 'uni',0 ).'

Will give you:

strout = 
    'a=f;'
    'b=f;'
    'c=f;'
    'd=f;'
    'e=f;'

Note: As Horchler rightly pointed out in comment, although the cellfun instruction allows to compact your code, it is just a disguised loop. Moreover, since it runs on cell, it is notoriously slow. You won't see the difference on such simple inputs, but keep this use when super performances are not a major concern.

Now if you like regex you must like black magic code. If all your strings are in a cell array from the start, there is a way to (over)abuse of the cellfun capabilities to ~~obscure your code~~ do it all in one line.

Consider:

strlist = {
'a=b=c=d;'
'a=b;'
'a=b=c=d=e=f;'
};

Then you can have all your substring with:

strout = cellfun( @(s)cellfun(@(a,b)cat(2,a,'=',b),s(1:end-1),repmat(s(end),1,length(s)-1),'uni',0).' , cellfun(@(s) strsplit(s,'=') , strlist , 'uni',0 ) ,'uni',0)
>> strout{:}
ans = 
    'a=d;'
    'b=d;'
    'c=d;'
ans = 
    'a=b;'
ans = 
    'a=f;'
    'b=f;'
    'c=f;'
    'd=f;'
    'e=f;'

This gives you a 3x1 cell array. One cell for each group of substring. If you want to concatenate them all then simply: strall = cat(2,strout{:});

I think that you can simplify your `cellfun` line in this case: `strout = cellfun(@(a)[a,'=',list{end}], list(1:end-1), 'uni', 0).'`. And I wouldn't necessarily call `cellfun` avoiding a `for` loop as it's kind of a loop in disguise – it's a convenience, but not necessarily faster than just using a simple loop. — horchler, May 01 '15 at 22:34
@horchler. Thanks, well spotted. I corrected the post giving you credit and also highlighted the slowness of `cellfun`. I didn't managed to apply your simplification to the global solution I posted in edit though :( — Hoki, May 01 '15 at 23:48
@Hoki thanks for your reply. But my problem is not to break a string down into different pieces only. My task is first to find the pattern in a long string. For example, I want to find a pattern like "X=Y=...=Z" and then replace it with smaller strings. Your solution assumes that you already have a string with the pattern "X=Y=...=Z". For pattern search, I needed to use regexprep. — ubaabd, May 02 '15 at 10:57
@ubaabd. Indeed, my solution won't help you for that. Sorry about it. But your question wasn't mentioning what you just said in comment, it started with "_I **have** the following strings:_" ... hence my assumption. You should edit your question and mention that important fact. — Hoki, May 02 '15 at 11:41

score 1 · Answer 2 · answered May 01 '15 at 21:30

I haven't had much experience w/ Matlab; but your problem can be solved by a simple string split function.

[parts, m] = strsplit( funstr, {' ', '='}, 'CollapseDelimiters', true )

Now, store the last part of parts; and iterate over parts until that:

len = length( parts )
for i = 1:len-1
    print( strcat(parts(i), ' = ', parts(len)) )
end

I do not know what exactly is the print function in matlab. You can update that accordingly.

score 0 · Answer 3 · edited May 23 '17 at 11:51

There isn't a single Regex that you can write that will cover all the cases. As posted on this answer:

https://stackoverflow.com/a/5019658/3393095

However, you have a few alternatives to achieve your final result:

You can get all the values in the line with regexp, pick the last value, then use a for loop iterating throughout the other values to generate the output. The regex to get the values would be this:

matchStr = regexp(str,'([^=;\s]*)','match')
If you want to use regexprep at any means, you should write a pattern generator and a replace expression generator, based on number of '=' in the input string, and pass these as parameters of your regexprep func.
You can forget about Regex and Split the input to generate the output looping throughout the values (similarly to alternative #1) .

Dynamic regexprep in MATLAB

3 Answers3