MATLAB: How to make the desired Regular Expression

Question

I am trying to use regexprep to remove certain parts of strings that meet a specific criteria. Here are the criteria in the best English I can think of:

Remove the ## if it occurs at the beginning of the first word even if this word is indented by whitespace.

And here is what I have so far. It works for most of the test cases but fails the last two:

%Test string
str = {'##hello','h##ello','  ##hello','hello','####hello',...
  '  ####hello','h## ello','##','h  ##ello','##hello ##hello'}';
%Match at beginning of word and look behind for whitespace
regs = '(?<!\S)\>(##)';
str2 = regexprep(str,regs,'');    
%What I actually want
str3 = {'hello','h##ello','  hello','hello','##hello',...
   '  ##hello','h## ello','','h  ##ello','hello ##hello'}';
%Pretty visualization
ds = dataset(str,str2,str3,'VarNames',{'Input','Actual_Results','Wanted_Results'})

So it works for all of the cases where there is not a word in front of the ##. However if there is text behind the space, the lookbehind doesn't pick this up. Also I can't use the match beginning of line because I want it to be able to handle n-length white space which beginning of line counts.

Answer 1

This seems to work for your test case:

str4 = regexprep(str,'^(\s*)(#{2})','$1');

Or, even simpler:

str4 = regexprep(str,'^(\s*)(##)','$1');