Hey guys, my problem involves rewriting equations with regular expressions. I have a list of special "endogenous" variables which need to be changed from "x" to "xSS*exp(x)" everywhere they occur in an equation. I'm working with time series, so "x(-1)" or "x(+1)" needs to be changed to "xSS*exp(x(-1))" or "xSS*exp(x(+1))", as well.

For example, let's suppose that my endogenous variables are:

y, c, k, a, and h.

I need to take this:

y = a*(k(-1)ˆalpha)*(hˆ(1-alpha));

and turn it into this:

(ySS*exp(y)) = (aSS*exp(a))*((kSS*exp(k(-1)))ˆalpha)*((hSS*exp(h))ˆ(1-alpha));

Now I've already gotten started on this. Assume I already have a list of variables and equations to work with (contained in a single string which also has new lines). My code is:


my $model = "c*theta*hˆ(1+psi)=(1-alpha)*y;\n1 = beta*(c/c(+1)*alpha*y
+(+1)/k+(1-delta));\ny = a*(k(-1)ˆalpha)*(hˆ(1-alpha));\nk = y-c+(1-de
+lta)*k(-1);\nln(a) = rho*ln(a(-1))+ e;\n";

my @varlist = ('y','c','k','a','h');

my ($replacement,$search_string);
foreach my $var (@varlist){
      $search_string = quotemeta($var);
      $replacement = "$steady_states{$var}*exp(${var})";
      $model =~ s/(\W)$search_string(\W)/$1($replacement)$2/g;
      $model =~ s/^$search_string(\W)/($replacement)$1/g;
      $model =~ s/exp\($search_string(\))+(\([\+\-]1\))/exp($search_st
+ring$2$1)/g;
    }

print $model;

[download]

And that's mostly okay except for the equation where you find "c/c(+1)" because the "/" character is already captured when the code finds "c/" - therefore "/c(+1)" isn't replaced. You can't just ignore the first (\W) because then parameters like "theta" would be problematic.

I know my regexs aren't very good anyways, so I'd like some advice about how to handle this problem.

Thank you very much for your time - hopefully I've explained everything clearly.

In reply to Help with regex - find captured pattern twice by pachydermic

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