Why Does a Repeated Capture Group Return these Strings?

by

Repeated Capture Group: The Group Number Stays the Same

The group defined by (..) is Group 1. The + quantifier repeats it. Every time the engine is able to repeat the group (matching two characters), Group 1 gets overwritten.

  • When the engine starts to match, it captures aa to Group 1
  • It then captures bb to Group 1
  • It then captures cc to Group 1.

When you inspect Group 1, the engine returns cc. All other captures are lost.

(The exception is the .NET engine, which also returns cc but also allows you to inspect intermediate captures thanks to the CaptureCollection object. It would contain aa, bb and cc.)

With (..)+(...), Why does Group 1 Contain aa? Backtracking!

To understand this, we again need to follow the path of the regex engine.

  • Once again, when the engine starts to match, it captures aa to Group 1
  • Again, it repeats the (..) group and captures bb to Group 1
  • Again, it repeats the (..) group and captures cc to Group 1
  • The engine now tries to match (...). It fails: there are no characters left to consume.
  • The engine backtracks both in the string and in the regex pattern. The + means one or more times, and we matched .. three times, so we can give one up, or even two. At this stage, the engine gives up the last match of the quantified (..)+ group, which is cc. We are back to when Group 1 was bb.
  • The engine tries to match (...) again. There are only two characters left: cc, so it fails again.
  • The engine backtracks by giving up the last match of the quantified (..)+ group, which is bb. At this stage, Group 1 is aa again.
  • The engine tries to match (...) again. It succeeds: Group 2 is bbc, and Group 1 is aa

Reference

More Related Contents:

  1. How to use a variable inside a regular expression?
  2. Match a whole word in a string using dynamic regex
  3. How to check for valid email address? [duplicate]
  4. Python string.replace regular expression [duplicate]
  5. Split a string at uppercase letters
  6. Regex matching 5-digit substrings not enclosed with digits
  7. How to split but ignore separators in quoted strings, in python?
  8. Python regex match OR operator
  9. Regular expression to extract URL from an HTML link
  10. BeautifulSoup – search by text inside a tag
  11. Python Regex escape operator \ in substitutions & raw strings
  12. How to extract an IP address from an HTML string?
  13. Get the string within brackets in Python
  14. Regex for existence of some words whose order doesn’t matter
  15. Split string based on a regular expression
  16. Split string on commas but ignore commas within double-quotes?
  17. Python/Regex – How to extract date from filename using regular expression?
  18. How would I get everything before a : in a string Python
  19. Python regex: splitting on pattern match that is an empty string
  20. Match any unicode letter?
  21. Python replace string pattern with output of function
  22. How can I make multiple replacements in a string using a dictionary?
  23. Python regex split without empty string
  24. Creating a custom categorized corpus in NLTK and Python
  25. Warning raised by inserting 4-byte unicode to mysql
  26. How to use str.contains() with multiple expressions in pandas dataframes
  27. match dates using python regular expressions
  28. How to Fix JSON Key Values without double-quotes?
  29. python regular expression across multiple lines
  30. How to use regex with optional characters in python?

Leave a Comment