How to use regexp to find strings in cell array beginning and ending with a certain substring?

Question

Consider the following cell array of strings containing filenames:

A = { 'abcout.txt';
      'outabcd.txt';
      'outabcef.png';
      'outout.txt' }

I'd like to find all .txt-files starting with "out".

I could do it as following:

filenames = regexp( A ,'out\w*.txt');
filenames = A( cellfun(@(x) ~isempty(x) && x == 1,filenames) )

returning the desired output:

filenames = 

    'outabcd.txt'
    'outout.txt'

But I wonder how I could use regexp to skip the cellfun step?

The following almost works:

filenames = regexp( A ,'out\w*.txt','match');
filenames = [filenames{:}]'

but it returns also the first string, which is invalid (and not even correctly displayed):

filenames = 

    'out.txt'
    'outabcd.txt'
    'outout.txt'

How do I need to modify: 'out\w*.txt' ?

Rody Oldenhuis · Accepted Answer · 2014-03-19 13:40:52Z

2

Personally, I would modify that to

'^out.*\.txt$'

because \w* excludes things like

'out file.txt'

which I think should be included...Moreover, the original string is incorrect, in that it also matches

'outFileWtxt'

because you've not escaped the . metacharacter :)

Anyway, from a performance standpoint, getting rid of cellfun is actually not what you want; you should just use it properly:

%// dummy data
A = { 'abcout.txt';
      'outabcd.txt';
      'outabcef.png';
      'outout.txt' };

%// Make sure we have something substantial to do
A = repmat(A, 1e5,1);

%// New way   
tic
    F = regexp(A, '^out.*\.txt$', 'match');
    F = [F{:}];
toc

%// Old way with optimized cellfun() call
tic
    F = regexp(A, '^out.*\.txt$');
    F = A(~cellfun('isempty', F));
toc

Results:

Elapsed time is 0.928403 seconds. %// without cellfun
Elapsed time is 0.471774 seconds. %// with optimized cellfun

This call to cellfun is faster because the string options refer to specific, hard-coded functions in the cellfun binary. This is a lot faster than any anonymous function will be, because that has to be evaluated back in the MATLAB environment.

answered Mar 19, 2014 at 13:40

Rody Oldenhuis

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1 Comment

Robert Seifert Over a year ago

Thanks for the additional advices, I was not considering so much exceptions before.

Sabuj Hassan · Accepted Answer · 2014-03-19 13:28:00Z

2

Use ^ to anchor at the beginning of the string and $ at the end of the string.

filenames = regexp( A ,'^out\w*.txt$');

Right now the out.txt you got from the text abcout.txt as you didn't use anchor.

answered Mar 19, 2014 at 13:28

Sabuj Hassan

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Comments

pezetem · Accepted Answer · 2014-03-19 13:29:21Z

1

try this regex: ^ - states for the beginning of the line

^out\w*.txt

answered Mar 19, 2014 at 13:29

pezetem

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