Advanced regex: Can't figure a case where (?R) recursive regex can be used

Question

Is anyone here can explain a case where the recursive Perl/PCRE regex (?R) can be helpful ?

I read

• https://regular-expressions.mobi/recursebacktrack.html?wlr=1
• https://perldoc.perl.org/perlre.html#(%3fPARNO)-(%3f-PARNO)-(%3f%2bPARNO)-(%3fR)-(%3f0)

but still can't find a use-case.

Answer 1

The canonical example is matching an arithmetical expression containing numbers, operators and subexpressions in parentheses:

([0-9]+|\((?R)\))([-+*\/]([0-9]+|\((?R)\)))*
 ^^^^^^ ^^^^^^^^  ^^^^^^^ ^^^^^^ ^^^^^^^^
 number (subexp)   oper   number (subexp)
\______  _______/        \______  _______/
       \/                       \/
  One of those             One of those
                  \__________  ____________/
                             \/
                     Zero or more of those

that is, match

• Either a number or a subexpression in parentheses, followed by
• Zero or more times an operator and either a number or a subexpression in parentheses.

Answer 2

That is the tool to use to match nested structures, like nested parenthesis.
The mathematical defined regular expressions can not match this.

\(  ( [^()]*+ | (?R) )*  \)

View it online (spaces should be ignored).

Please read:

• Matching Balanced Constructs
• Using Recursion to Match Palindromes (mirror words)