General ⇒ Parsing a String using Regular Expressions in LaTeX?!

[foobar]

Hi everyone! This is the code I have:

I have two counters defined

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\newcounter{a}
\newcounter{b}

and a formula

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(a+b)/2

given to the following command (as #1).

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\expandafter\parseValue#1
\setcounter{c}{((\value{\a}+\value{\b})*10/\value{divisor}+5)/10}

The command "parses" the formula (#1) using \expandafter and \parseValue.

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\def\parseValue(#1+#2)/#3{%
  \def\a{#1}
  \def\b{#2}
  \setcounter{divisor}{#3}
}

I now want to handle other formulas (e.g. (a+b+c)/3) with the same code. So I need to get something like a Regular Expression in (La)TeX in order to distinguish between different formulas.

And these are my questions:

1) Is any package out there dealing with Regular Expressions?
2) a. Or do I have to hack my own code using any String Manipulation Package?
2) b. Do you have any experience which packets fit best?
3) Or does a package already deal with the issue I'm working on?

Thanks in advance!

[josephwright]

Assuming you use e-TeX, why not just

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\setcounter{c}{\numexpr{(\value{a}+\value{b})/2}}
[\code]
and so forth?

[foobar]

I didn't know the \numexpr command and it is really a much cleaner way to round a number. Thanks for that! But my problem remains the same:

I want to avoid using \value and instead just use the counters name. There are just two types of formulas I want to use:

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1. (a+b)/2
2. (a+b+c)/2

All addends will always be counter names. I would just have to extract and wrap them into the \value command.

Oh, and I think I just found a solution for that! Redefining my parser to

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\def\parseValue(#1)/#2{%
  \def\sum{#1}
  \setcounter{divisor}{#2}
}

and splitting the \sum using a slightly modified version of this code to split a string on plus signs rather than commas will do the job I guess...

Thanks for your post and the \numexpr command! (And maybe there is still a better solution?)

[josephwright]

While there are general solutions (making everything active), you need to remember that as far as TeX is concerned macros, counters and so on need to start with the escape character (\). LaTeX makes counters with internal names \c@<name>, so \newcounter{a} makes a TeX count \c@a. The type of thing you want to do is really not the TeX way (I think you might be best using LuaTeX if you are really determined).