How to neatly match "x" and "[x]" with a regex without repeating?

Question

I'm writing a Perl regex to match both the strings x bla and [x] bla. One alternative is /(?:x|\[x\]) bla/. This isn't desirable, because in the real world, x is more complicated, so I want to avoid repeating it.

The best solution so far is putting x in a variable and pre-compiling the regex:

my $x = 'x';
my $re = qr/(?:$x|\[$x\]) bla/o;

Is there a neater solution? In this case, readability is more important than performance.

Answer 1

It's possible, but not all that clean. You can use the fact that conditional subpatterns support tests such as (?(N)) to check that the Nth capturing subpattern successfully matched. So you can use an expression such as /(\[)?X(?(1)\])/ to match '[X]' or 'X'.

Answer 2

You can pre-compile $x as well. This also makes errors a little more obvious if $x is really ?(+[*{ or something else that a regex compiler will completely freak out on.

my $x = qr/x/;
my $re = qr/(?:$x|\[$x\]) bla/o;

Answer 3

There isn't a neater solution really, because this is where we leave the domain of regular languages and start requiring a more complex automaton with some kind of memory. (Backrefs would do it, except that the backref expands to a literal match against a preceding part of the string, not to “this, but only if that was matched”.)

Sometimes, it's possible to instead use a two step process, replacing a complex X with a single character known to not be present in the source text (control characters can be suitable for that) so allowing a simpler second-stage match. Not always possible though; depends on what you're matching.