Capturing Quantifiers and Quantifier Arithmetic

Question

At the outset, let me explain that this question is neither about how to capture groups, nor about how to use quantifiers, two features of regex I am perfectly familiar with

Answer 1

I don't know a regex engine that can capture a quantifier. However, it is possible with PCRE or Perl to use some tricks to check if you have the same number of characters. With your example:

@@@@ "Star Wars" ==== "1977" ---- "Science Fiction" //// "George Lucas"

you can check if @ = - / are balanced with this pattern that uses the famous Qtax trick, (are you ready?): the "possessive-optional self-referencing group"

~(?

pattern details:

~                          # pattern delimiter
(?

The main idea

The non-capturing group contains only one @. Each time this group is repeated a new character is added in capture groups 2, 3 and 4.

the possessive-optional self-referencing group

How does it work?

( (?: @ (?= [^=]* (\2?+ = ) .....) )+ )

At the first occurence of the @ character the capture group 2 is not yet defined, so you can not write something like that (\2 =) that will make the pattern fail. To avoid the problem, the way is to make the backreference optional: \2?

The second aspect of this group is that the number of character = matched is incremented at each repetition of the non capturing group, since an = is added each time. To ensure that this number always increases (or the pattern fails), the possessive quantifier forces the backreference to be matched first before adding a new = character.

Note that this group can be seen like that: if group 2 exists then match it with the next =

( (?(2)\2) = )

The recursive way

~(?[^@=]+|(?-1))*=)(?!=))(?=(@(?>[^@-]+|(?-1))*-)(?!-))(?=(@(?>[^@/]+|(?-1))*/)(?!/))~

You need to use overlapped matches, since you will use the @ part several times, it is the reason why all the pattern is inside lookarounds.

pattern details:

(?           # open an atomic group
            [^@=]+    # all that is not an @ or an =, one or more times
          |           # OR
            (?-1)     # recursion: the last defined capturing group (the current here)
        )*            # repeat zero or more the atomic group
        =             #
    )                 # close the capture group
    (?!=)             # checks the = boundary
)                     # close the lookahead
(?=(@(?>[^@-]+|(?-1))*-)(?!-))  # the same for -
(?=(@(?>[^@/]+|(?-1))*/)(?!/))  # the same for /

The main difference with the precedent pattern is that this one doesn't care about the order of = - and / groups. (However you can easily make some changes to the first pattern to deal with that, with character classes and negative lookaheads.)

Note: For the example string, to be more specific, you can replace the negative lookbehind with an anchor (^ or \A). And if you want to obtain the whole string as match result you must add .* at the end (otherwise the match result will be empty as playful notices it.)