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

but still can't find a use-case.

score 2 · Answer 1 · answered Jun 29 '20 at 23:58

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.

score 1 · Accepted Answer · answered Jun 30 '20 at 05:07

1

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:

answered Jun 30 '20 at 05:07

`perl -lne 'print $& if /$ ( [^()]*+ | (?R) )* $/gmx' file` Very nice trick ! – Mévatlavé Kraspek Jul 01 '20 at 00:04

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

2 Answers2