Replacing substrings in Matlab

Question

In Matlab I have a string, that represents a function. Let it be something like that '...sin(arcsin(f_2))...'. I want to find all appearances of 'sin(arcsin(F))' for any F and replace it with simple 'F'.

I don't know what kind of function if F. It may be 'exp' or 'cos' or something else.

How can I do it? Is it any smart way to do it without while-loop?

There is a built in function for that: http://www.mathworks.com/help/matlab/ref/strrep.html — Dan, Mar 17 '14 at 13:27
How do you generate these strings? Can't it be easier to prevent the creation of this (and similar) constructions in the first place? I suspect you could use something like `simplify` here to get rid of it... — Rody Oldenhuis, Mar 17 '14 at 13:56
@Rody Oldenhuis, you're right, but it was supposed to be true — Macaronnos, Mar 17 '14 at 14:04
@restrest: Nevermind, I'm actually not so sure about the inequality. I numerically verified my statement, but then I started suspecting I was looking at numerical artefacts rather than mathematical truths... — Rody Oldenhuis, Mar 17 '14 at 14:05
@Rody Oldenhuis, in this way I won't be able do it for generated strings:) — Macaronnos, Mar 17 '14 at 14:14
@restrest: If you have the symbolic toolbox, you can also use `char(sym(str))`; apparently, this automatically gets rid of self-inverted things like this. — Rody Oldenhuis, Mar 17 '14 at 14:17

score 2 · Accepted Answer · edited May 23 '17 at 10:25

There's a couple of ways:

>> char(sym(str))  %// requires symbolic toolbox

>> regexprep(str, 'sin\(arcsin\(F\)\)', 'F')

>> strrep(str, 'sin(arcsin(F))', 'F')

The nice thing about the regexprep method is that you can be a lot more flexible with regard to spacing and casing:

>> regexprep(str, 'sin\s*\(\s*a(rc)*sin\s*\(\s*F\s*\)\s*\)', 'F', 'ignorecase')

the call above will convert all of the following:

>> str = ' sin (    arcsin(F )  )'
>> str = 'sin(arCSin(f) )'
>> str = '   Sin             (arcsin(f)  )       )'
>> str = 'Sin(Asin(f)))'

etc.

Note that for both methods above: str may be either a string or cell array of strings.

EDIT

You have indicated that F may be anything. That complicates matters. For simple matches you could still use something like

>> regexprep(str, 'sin\s*\(\s*a(rc)*sin\s*\(\s*(.+)\s*\)\s*\)', '$2', 'ignorecase')

But you'll have to be careful, because things like

>> str = 'acos(log( sin(arcsin( exp(x) )) * sin(x) ))'

will be incorrectly converted :

>> regexprep(str, 'sin\s*\(\s*a(rc)*sin\s*\(\s*(.+)\s*\)\s*\)', '$2', 'ignorecase')
ans = 
    acos(log( exp(x) )) * sin(x)

(note the incorrect bracketing). One solution is to use lazy operators:

>> regexprep(str, 'sin\s*\(\s*a(rc)*sin\s*\(\s*(.+?)\s*\)\s*\)', '$2', 'ignorecase')

but beware that you're really crossing over the border of what regular expressions can (or should) be used for. Not all possible cases will be handled successfully this way...See also this legendary answer

The best way by far is to prevent these strings from being generated that way in the fist place. I suspect that a simple sym/simplify will do away with this whole problem.

but how to make it works for any F. I don't know the appearance of F. It may be exponent or cos or smth — Macaronnos, Mar 17 '14 at 13:40
@restrest: Then you'll have to do something else; see my edits. — Rody Oldenhuis, Mar 17 '14 at 13:43

Replacing substrings in Matlab

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