Replacing numerics in text using regular expressions.

10 views (last 30 days)
Hello, I am trying to figure out whether it is possible to dynamically replace numeric values in a long text block using regular expressions. Here is an example from a made-up xml file.
str = '<document><placemark><when>5</when><lat>41</lat></placemark><placemark><when>11</when></placemark></document>';
Now, I want to perform some numerical function on all of the outputs of <when>, lets say subtract 3 so that the string will read
'<document><placemark><when>2</when><lat>41</lat></placemark><placemark><when>8</when></placemark></document>';
I can already find the locations using
exp='<when>(\d+)</when>'
but I don't know how to
  1. extract the actual numerical value at that location,
  2. perform some arbitrary function on that value (subtraction, addition, division, anything)
  3. write that new value back into the string so it reads <when>newValue</when>
If I was certain that the number of characters would stay the same, I could do a for-loop with some pretty gross indexing. However, as in the above example the length of the charstring representing the numeric value might change as a result of my function (11 became 8).
I suspect there is either a really elegant regexp solution, or it is not possible at all. Hoping for the former. Cheers, Dan
  3 Comments
Cedric Wannaz
Cedric Wannaz on 28 Sep 2017
Please see my last edit with a more "classical" approach.

Sign in to comment.

Accepted Answer

Cedric Wannaz
Cedric Wannaz on 28 Sep 2017
Edited: Cedric Wannaz on 28 Sep 2017
Here is a small trick assuming that no character in your string interferes with formatSpec stuff:
pattern = '(?<=<when>)\d+' ;
values = str2double( regexp( str, pattern, 'match' )) ;
values = values - 3 ; % Some operation.
fSpec = regexprep( str, pattern, '%d' ) ; % Make input str a formatSpec ;)
newStr = sprintf( fSpec, values ) ;
Got to run, I will answer more tonight if you don't get a better answer.
Ok, got 5 more minutes, otherwise a good alternative was mentioned by Walter and is based on the fact that you can run MATLAB code within the replacement pattern:
repFun = @(s) sprintf( '%d', sscanf( s,'%d' ) - 3 ) ; % Update function.
newStr = regexprep( str, '(?<=<when>)\d+', '${repFun($0)}' ) ;
Finally a more "classical" approach, that matches and splits the input string, replaces the matches and rebuilds the output.
[numbers, parts] = regexp( str, '(?<=<when>)\d+', 'match', 'split' ) ;
numbers = arrayfun( @(x) sprintf( '%d', x ), str2double( numbers ) - 3, ...
'UniformOutput', false ) ;
buffer = [parts; [numbers, {''}]] ;
newStr = sprintf( '%s', buffer{:} ) ;
  2 Comments
D. Plotnick
D. Plotnick on 28 Sep 2017
Thanks! Both to you and Walter. This worked very well, and I was hoping that I would be able to run MATLAB functions in this context, so that opens up a whole bunch of additional ways I can use this method.
Thanks again, Dan

Sign in to comment.

More Answers (1)

Walter Roberson
Walter Roberson on 28 Sep 2017
Yes. If you can devise a regexp pattern to isolate the number, then you can use regexprep with the ${cmd} replacement. Arguments to the commands will be passed as strings. Values can be returned as strings or as integers that will be converted to strings.
For example,
str = '<document><placemark><when>5</when><lat>41</lat></placemark><placemark><when>11</when></placemark></document>';
regexprep(str, '\d+', '${$0 - 2}')
I did not test this code (my system is busy at the moment)
  2 Comments
Cedric Wannaz
Cedric Wannaz on 28 Sep 2017
You could have done it by coding the conversion to double as well. $0 refers to the match, which is a string. It must be converted to double before you can do math. Instead of loading the replacement string with commands, I created a function repFun that we call, and this function does the double conversion string-num-string.

Sign in to comment.

Categories

Find more on Characters and Strings in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!