How can I recursively match a pattern using Regular Expressions?

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The string can be like one of the following:


I want to match an unlimited number of "a(x,y)". How can I do that using Regex? Here's what I have:


It only matches up two recursions of "a(x,y)".

Java's standard regex lib does not support recursion, so you can't match such general nested constructs with it.

But in flavors that do support recursion (Perl, PCRE, .NET, etc) you can use expressions like:


Regular Expression Recursion, Since these regexes are functionally identical, we'll use the syntax with R for recursion to see how this regex matches the string aaazzz. First, a matches the first� Now you have two carets preceding two distinct characters. This pattern can never match. Fortunately, you can build a recursive expression without using (?R), which repeats then entire expression. Instead, using the "subroutine expression" (or sub-pattern) syntax, you can paste a sub-pattern specified by a capture group. For instance, to paste

You can also use my Regular Expression library , that supports Recursive Regular Expressions! The API is basically the same as the one of java.util.regex, just the required import statements are different, e.g.

Pattern p = Pattern.compile("(?<first>a\\((?<second>(?'first')|[a-zA-Z]),(?'second')\\))");
assert p.matcher("a(a(a(a(a(b,c),d),a(e,f)),g),h)").find();

How can I recursively match a pattern using Regular Expressions , 2 options - 1) Use Lexical Analysis to do the pattern matching & replacement on your own [OR] 2) If you want to stick to Regex then use some shell programming (or any supporting language) & call it from Java. Not sure how you can do in a language. Java provides the java.util.regex package for pattern matching with regular expressions. Java regular expressions are very similar to the Perl programming language and very easy to learn. A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a

2 options - 1) Use Lexical Analysis to do the pattern matching & replacement on your own [OR] 2) If you want to stick to Regex then use some shell programming (or any supporting language) & call it from Java.

Pattern Matching and Recursion, R)\))+ matches balanced parentheses, and for people who understand regular expressions well, the shape of the expression matches the shape� Introduction. The grep command is one of the most useful commands in a Linux terminal environment. The name grep stands for “global regular expression print”. This means that you can use grep to see if the input it receives matches a specified pattern.

I think you are looking for something like:

a(x,y) = [a-z] ( [a-z] , [a-z] )

regex = a(x,y) | a(regex|y) | a(x, regex)

Not sure how you can do in a language.

Recursive Regular Expression, To use recursive regex, you use (?R) or (?0) . When the regex This tells the engine to attempt the whole regex again at the present position in the string. 1 match a 3 or 4 characters palindrome. eg: 'kaak' or 'kak'. With� While line-oriented tools from Unix such as sed and grep match regular expressions one line at a time, not all tools do. The sam text editor from Bell Laboratories was the first interactive tool I know of to allow multiline regular expressions; the Perl scripting language followed shortly. In the Java API, the newline character by default has

The language you're describing isn't a regular language so it can't be matched by a regular expression. Look into lexical analysis (i.e. use a parser)

Crucial Concepts Behind Advanced Regular Expressions , The syntax used here matches PHP regular expressions. They require the use of (?R) , an operator that matches recursive sub-patterns. What are Regular Expressions? Regular expressions specify patterns to search for in string data using standardized syntax conventions. A regular expression can specify complex patterns of character sequences. For example, the following regular expression: a(b|c)d searches for the pattern: 'a', followed by either 'b' or 'c', then followed by 'd'.

Recursive Regular Expressions, For instance, the so often used capture buffers add auxiliary storage to the regular expressions that allow them to match an arbitrary pattern repeatedly. 8.1 Overview of Regular Expressions. A regular expression specifies a search pattern, using metacharacters (which are, or belong to, operators) and character literals (described in Oracle Database SQL Language Reference). The search pattern can be complex.

Recursive Regular Expressions, Nesting is not necessary, you can iterate instead But first: naming the input String string can be confusing. Of course it's a string, what is it used for though? Note. By default, the (subexpression) language element captures the matched subexpression. But if the RegexOptions parameter of a regular expression pattern matching method includes the RegexOptions.ExplicitCapture flag, or if the n option is applied to this subexpression (see Group options later in this topic), the matched subexpression is not captured.

Recursive patterns - Manual, Consider the problem of matching a string in parentheses, allowing for unlimited The recursion in regular expressions is the only way to allow the parsing of� 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 {#} .

  • See this answer:
  • Are you sure Python has such a feature? AFAIK, only PHP, .NET and Perl are capable of such trickery.
  • @BartKiers, correct, Python does not have (?R) and friends, for some reason I thought that newer versions of its regex engine did support it. Removed it from the list.
  • I'm not too familiar with Python, so I thought they included it in v3 (which I haven't really looked at).
  • Maybe you're thinking of Ruby. Since version 1.9 it has used the Oniguruma regex library, which does support recursion.
  • @AlanMoore, Oniguruma was the one I was thinking about, thanks for reminding me.
  • doesr not suppor ?R
  • ...this is not an answer.
  • -1. Regex (in this context) are not regular, and they haven't been for 25+ years now.
  • Nitpick: lexical analysis is based on regular expressions; lexical analysis turns a stream of characters into a stream of symbols according to rules defined by regular expressions. The thing that solves the problem that the asker asks about is parser generators.
  • @robert, (obviously?) Qtax meant regex in OS-tools/programming API's. Regex in that context have had the capability to match something that had been matched before (grouping and back-referencing): making them able to match more than regular languages.
  • The point is, simply saying "that's not a regular language" doesn't help. We're talking about regexes, not theory-pure regular expressions. Some regex flavors support recursion, but Java doesn't happen to be one of them.