Can regular expressions be used to match nested patterns

Question

Is it possible to write a regular expression that matches a nested pattern that occurs an unknown number of times  For example  can a regular expression match an opening and closing brace when there are an unknown number of open close braces nested within the outer braces   For example   public MyMethod       if  test             More               More          End   Should match       if  test             More               More

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No  you are getting into the realm of Context Free Grammars at that point

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YES     assuming that there is some maximum number of nestings you d be happy to stop at   Let me explain      torsten-marek is right that a regular expression cannot check for nested patterns like this  BUT it is possible to define a nested regex pattern which will allow you to capture nested structures like this up to some maximum depth  I created one to capture EBNF-style comments  try it out here   like      This is a comment    this is nested inside    another level     hey    yo      The regex  for single-depth comments  is the following   m 1                                                       This could easily be adapted for your purposes by replacing the        and        with   and   and replacing everything in between with a simple         p 1                      Here s the link to try that out    To nest  just allow this pattern within the block itself   p 2            p 1                      or    p 2                                            To find triple-nested blocks  use   p 3            p 2                      or    p 3                                                                   A clear pattern has emerged  To find comments nested to a depth of N  simply use the regex   p N            p N-1                    where N  gt  1 and   p 1                     A script could be written to recursively generate these regexes  but that s beyond the scope of what I need this for   This is left as an exercise for the reader

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This seems to work                           m

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Probably working Perl solution  if the string is on one line   my  NesteD    NesteD   qr                    NesteD           x    if    Stringy    m  b   w  NesteD   x         print  Found   1 n          HTH  EDIT  check    http   dev perl org perl6 rfc 145 html ruby information  http   www ruby-forum com topic 112084 more perl  http   www perlmonks org  node id 660316 even more perl  https   metacpan org pod Text  Balanced perl  perl  perl  http   perl plover com yak regex samples slide083 html   And one more thing by Torsten Marek  who had pointed out correctly  that it s not a regex anymore     http   coding derkeiler com Archive Perl comp lang perl misc 2008-03 msg01047 html

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as zsolt mentioned  some regex engines support recursion -- of course  these are typically the ones that use a backtracking algorithm so it won t be particularly efficient   example      gt            gt          R     gt          sm

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No  It s that easy  A finite automaton  which is the data structure underlying a regular expression  does not have memory apart from the state it s in  and if you have arbitrarily deep nesting  you need an arbitrarily large automaton  which collides with the notion of a finite automaton   You can match nested paired elements up to a fixed depth  where the depth is only limited by your memory  because the automaton gets very large  In practice  however  you should use a push-down automaton  i e a parser for a context-free grammar  for instance LL  top-down  or LR  bottom-up   You have to take the worse runtime behavior into account  O n 3  vs  O n   with n   length input    There are many parser generators avialable  for instance ANTLR for Java  Finding an existing grammar for Java  or C  is also not difficult  For more background  Automata Theory at Wikipedia

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The Pumping lemma for regular languages is the reason why you can t do that   The generated automaton will have a  finite number of states  say k  so a string of k 1 opening braces is bound to have a state repeated somewhere  as the automaton processes the characters   The part of the string between the same state can be duplicated infinitely many times and the automaton will not know the difference   In particular  if it accepts k 1 opening braces followed by k 1 closing braces  which it should  it will also accept the pumped number of opening braces followed by unchanged k 1 closing brases  which it shouldn t

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Yes  if it is  NET RegEx-engine   Net engine supports finite state machine supplied with an external stack  see details

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Yes  if it is  NET RegEx-engine   Net engine supports finite state machine supplied with an external stack  see details

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Using regular expressions to check for nested patterns is very easy           gt          1

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Using the recursive matching in the PHP regex engine is massively faster than procedural matching of brackets  especially with longer strings   http   php net manual en regexp reference recursive php  e g    patt                   gt             R          x    preg match all   patt   str   m      vs   matchBrackets   str     function matchBrackets    str   offset   0           matches   array         list   opener   closer     array                      Return early if there s no match     if   false        first offset   strpos   str   opener   offset                 return  matches                Step through the string one character at a time storing offsets      paren score   -1       inside paren   false       match start   0       offsets   array         for    index    first offset   index  lt  strlen   str     index                char    str   index             if    opener      char                 if      inside paren                      paren score   1                   match start    index                            else                    paren score                               inside paren   true                    elseif    closer      char                  paren score--                     if   0      paren score                  inside paren   false               paren score   -1               offsets     array   match start   index   1                         while    offset   array shift   offsets                list   start   finish      offset            match   substr   str   start   finish -  start             matches      match             return  matches

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Proper Regular expressions would not be able to do it as you would leave the realm of Regular Languages to land in the Context Free Languages territories   Nevertheless the  regular expression  packages that many languages offer are strictly more powerful   For example  Lua regular expressions have the   b    recognizer that will match balanced parenthesis  In your case you would use   b     Another sophisticated tool similar to sed is gema  where you will match balanced curly braces very easily with       So  depending on the tools you have at your disposal your  regular expression   in a broader sense  may be able to match nested parenthesis

[regex] Can regular expressions be used to match nested patterns?

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