e =~ u|uae|edefe|ebe|ec
It should be doable in more up-to-date perl. The expression e must start with token u, so let's rewrite it using e => uE?
e =~ u|uauE?|uE?defe|uE?be|uE?c
E =~ auE?|E?defe|E?be|E?c
It should be evident that E? simplifies as
E =~ (au)*(defe|be|c)*

Putting things together: 

#! /usr/bin/perl

my $g3 = "( d(?1)f(?1) | b(?1) | c )";      # (defe|be|c)
my $g2 = "( (?:au)* (?3)* )";               # E?
my $g1 = "( u | uau(?2) | u(?2)(?3) )";     # e

my $re = qr/ $g1 (?(DEFINE) $g2 $g3 ) /x;

warn "$re\n";
/^$re$/ and print while <>;

[download]

But of course, this won't work with perl5.6 ...

Update. It looks this simplifies to just

my $re = qr/(u(?:au)*(?:d(?-1)f(?-1)|b(?-1)|c)*)/;

In reply to Re: When the only tool you have is a screwdriver... by oiskuu
in thread When the only tool you have is a screwdriver... by ExReg

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