Regular Expression Lookbehind doesn’t work with quantifiers (‘+’ or ‘*’)

by

Many regular expression libraries do only allow strict expressions to be used in look behind assertions like:

  • only match strings of the same fixed length: (?<=foo|bar|\s,\s) (three characters each)
  • only match strings of fixed lengths: (?<=foobar|\r\n) (each branch with fixed length)
  • only match strings with a upper bound length: (?<=\s{,4}) (up to four repetitions)

The reason for these limitations are mainly because those libraries can’t process regular expressions backwards at all or only a limited subset.

Another reason could be to avoid authors to build too complex regular expressions that are heavy to process as they have a so called pathological behavior (see also ReDoS).

See also section about limitations of look-behind assertions on Regular-Expressions.info.

More Related Contents:

  1. How does the regular expression ‘(?
  2. What’s the technical reason for “lookbehind assertion MUST be fixed length” in regex?
  3. Lookbehind on regex for VBA?
  4. Does lookbehind work in sed?
  5. Does lookaround affect which languages can be matched by regular expressions?
  6. How to match the first word after an expression with regex?
  7. RegEx for Multi-Words Without Numbers and Any Special Characters
  8. What is this pattern “\\s*\\^++\\s*” used for? [duplicate]
  9. Regular expression to match balanced parentheses
  10. How do I deal with special characters like \^$.?*|+()[{ in my regex?
  11. Regular expression that matches valid IPv6 addresses
  12. How to capture multiple repeated groups?
  13. Regular expression to enforce complex passwords, matching 3 out of 4 rules
  14. Regular expression to allow spaces between words
  15. Regex: Specify “space or start of string” and “space or end of string”
  16. Regex – Does not contain certain Characters
  17. Delete all content but keeping matched
  18. regex for zip-code
  19. Regex to match 2 digits, optional decimal, two digits
  20. Extracting top-level and second-level domain from a URL using regex
  21. HTML5 input pattern search for quote
  22. When should I not use regular expressions?
  23. regex to remove ordinals
  24. Replace a word with multiple lines using sed?
  25. Regular Expression Vs. String Parsing
  26. Can extended regex implementations parse HTML?
  27. How to get Vim to highlight non-ascii characters?
  28. Regexp for subdomain
  29. Match parenthesised block using regular expressions in vim
  30. RegEx for allowing alphanumeric at the starting and hyphen thereafter

Leave a Comment