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search and replace exact substring

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I am parsing a string to check for an exact phrase. If the exact phrase is found, then I want to replace the phrase with a new phrase. The problem is, sometimes the phrase will be part of anothe phrase. Example:
wish to change the phrase "eps" to "%eps"
str = 'steps = eps*100 + beta*steps*eps';
newStr = 'steps = %eps*100 + beta*steps*%eps';
I'm having trouble picking out the exact phrase; each parsing phrase that I've come up with always catches something undesired e.g. 'steps' => 'st%eps' etc.
I've tried things like: st1 = regexprep(str,'^\w*(?=eps)^\w*','%eps')
(where I think I'm saing, look around the phrase "eps", if there's no "word" in the regexp sense, then replace the phrase with "%eps", but I'm obviously not there yet.
CDQ (R2011b)


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Accepted Answer

Azzi Abdelmalek
Azzi Abdelmalek on 8 Oct 2014
Edited: Azzi Abdelmalek on 8 Oct 2014
str = 'steps = eps*100 + beta*steps*eps';
regexprep([' ' str ' '],'(?<=[^a-zA-Z])eps(?=[^a-zA-Z])','%eps')

  1 Comment

Notsure USA
Notsure USA on 8 Oct 2014
This appears to solve my problem; thank you Azzi. I had (probably still have) a fundamental misunderstanding of the lookahead/lookbehind operators.

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More Answers (1)

Image Analyst
Image Analyst on 8 Oct 2014
newString = strrep(oldString, ' eps', ' %eps');


Notsure USA
Notsure USA on 8 Oct 2014
Thank you for your answer; this was my original solution but unfortunately it doesn't catch one of the eps (the last one):
>> str = 'steps = eps*100 + beta*steps*eps';
>> newString = strrep(str, ' eps', ' %eps')
newString =
steps = %eps*100 + beta*steps*eps
Image Analyst
Image Analyst on 9 Oct 2014
You could do it in two lines of code instead of one:
newString = strrep(oldString, ' eps', ' %eps');
newString = strrep(newString, '*eps', '%eps');
Likewise in the event you have any other symbols. Sorry for not doing this in the first place - thought you'd know. I find this much easier, and more straightforward and intuitive than trying to compose/invent complicated regexp expressions, so that's why I suggested it. Looks like you're quite adept with regexp though and can understand things like ' (?<=[^a-zA-Z])eps(?=[^a-zA-Z]) ', and that's great, because not everybody can.

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