What is the meaning of + in a regex?

by

+ can actually have two meanings, depending on context.

Like the other answers mentioned, + usually is a repetition operator, and causes the preceding token to repeat one or more times. a+ would be expressed as aa* in formal language theory, and could also be expressed as a{1,} (match a minimum of 1 times and a maximum of infinite times).

However, + can also make other quantifiers possessive if it follows a repetition operator (ie ?+, *+, ++ or {m,n}+). A possessive quantifier is an advanced feature of some regex flavours (PCRE, Java and the JGsoft engine) which tells the engine not to backtrack once a match has been made.

To understand how this works, we need to understand two concepts of regex engines: greediness and backtracking. Greediness means that in general regexes will try to consume as many characters as they can. Let’s say our pattern is .* (the dot is a special construct in regexes which means any character1; the star means match zero or more times), and your target is aaaaaaaab. The entire string will be consumed, because the entire string is the longest match that satisfies the pattern.

However, let’s say we change the pattern to .*b. Now, when the regex engine tries to match against aaaaaaaab, the .* will again consume the entire string. However, since the engine will have reached the end of the string and the pattern is not yet satisfied (the .* consumed everything but the pattern still has to match b afterwards), it will backtrack, one character at a time, and try to match b. The first backtrack will make the .* consume aaaaaaaa, and then b can consume b, and the pattern succeeds.

Possessive quantifiers are also greedy, but as mentioned, once they return a match, the engine can no longer backtrack past that point. So if we change our pattern to .*+b (match any character zero or more times, possessively, followed by a b), and try to match aaaaaaaab, again the .* will consume the whole string, but then since it is possessive, backtracking information is discarded, and the b cannot be matched so the pattern fails.

1 In most engines, the dot will not match a newline character, unless the /s (“singleline” or “dotall”) modifier is specified.

More Related Contents:

  1. Reg-ex to validate a sting with digits only but all Zeros are not allowed [closed]
  2. RegEx for Multi-Words Without Numbers and Any Special Characters
  3. Regular expression to match balanced parentheses
  4. How do I deal with special characters like \^$.?*|+()[{ in my regex?
  5. Why doesn’t [01-12] range work as expected?
  6. Regular expression for a string containing one word but not another
  7. How do I use regex in a SQLite query?
  8. Regular expression that matches valid IPv6 addresses
  9. Regular expression to allow spaces between words
  10. Regular expression to limit number of characters to 10
  11. MongoDB Regex Search on Integer Value
  12. When is \G useful application in a regex?
  13. Regex – Does not contain certain Characters
  14. Regular expression for letters, numbers and – _
  15. How to replace all strings to numbers contained in each string in Notepad++?
  16. How .* (dot star) works? [closed]
  17. How do you extract IP addresses from files using a regex in a linux shell?
  18. regex for zip-code
  19. Using positive-lookahead (?=regex) with re2
  20. Extracting top-level and second-level domain from a URL using regex
  21. Regular expression limit string size
  22. HTML5 input pattern search for quote
  23. JavaScript: split doesn’t work in IE?
  24. regex to remove ordinals
  25. Regular Expression Vs. String Parsing
  26. How to get Vim to highlight non-ascii characters?
  27. Does lookaround affect which languages can be matched by regular expressions?
  28. Overlapping matches in R
  29. hgignore: help ignoring all files but certain ones
  30. Guide on how to use regex in Nginx location block section?

Leave a Comment