Match at every second occurrence

Question

Is there a way to specify a regular expression to match every 2nd occurrence of a pattern in a string   Examples   searching for a against string abcdabcd should find one occurrence at position 5 searching for ab  against string abcdabcd should find one occurrence at position 5 searching for dab  against string abcdabcd should find no occurrences searching for a against string aaaa should find two occurrences at positions 2 and 4

User · Answer

Suppose the pattern you want is abc d   You want to match the second occurrence of this pattern in a string   You would construct the following regex   abc d    abc d    This would match strings of the form     lt your-pattern gt     lt your-pattern gt    Since we re using the reluctant qualifier    we re safe that there cannot be another match of  between the two   Using matcher groups which pretty much all regex implementations provide you would then retrieve the string in the bracketed group which is what you want

User · Answer

Use grouping   foo    foo

User · Answer

If you re using C   you can either get all the matches at once  i e  use Regex Matches    which returns a MatchCollection  and check the index of the item  index   2    0    If you want to find the occurrence to replace it  use one of the overloads of Regex Replace   that uses a MatchEvaluator  e g  Regex Replace String  String  MatchEvaluator   Here s the code   using System  using System Collections Generic  using System Text  using System Text RegularExpressions   namespace ConsoleApplication1       class Program               static void Main string   args                        string input    abcdabcd                   Replace  second  a with m              string replacedString   Regex Replace                  input                   a                   new SecondOccuranceFinder  m   MatchEvaluator                Console WriteLine replacedString               Console Read                        class SecondOccuranceFinder                       public SecondOccuranceFinder string replaceWith                                 replaceWith   replaceWith                   matchEvaluator   new MatchEvaluator IsSecondOccurance                              private string  replaceWith               private MatchEvaluator  matchEvaluator              public MatchEvaluator MatchEvaluator                               get                                       return  matchEvaluator                                               private int  matchIndex              public string IsSecondOccurance Match m                                 matchIndex                    if   matchIndex   2    0                      return  replaceWith                  else                     return m Value

User · Answer

Would something like    pattern    pattern      work for you   Edit   The problem with this is that it uses the non-greedy operator     which can require an awful lot of backtracking along the string instead of just looking at each letter once  What this means for you is that this could be slow for large gaps

User · Answer

Back references can find interesting solutions here  This regex     a-z       1    will find the longest repeated sequence   This one will find a sequence of 3 letters that is repeated     a-z  3      1

User · Answer

There s no  direct  way of doing so but you can specify the pattern twice as in  a  a  a that match up to the second  a    The alternative is to use your programming language  perl  C        to match the first occurence and then the second one   EDIT  I ve seen other responded using the  non-greedy  operators which might be a good way to go  assuming you have them in your regex library

[regex] Match at every second occurrence

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