Order of regular expression operator (..|.. … ..|..)

by

Left to right, and the first alternative matched “wins”, others are not checked for. This is a typical NFA regex behavior. A good description of that behavior is provided at regular-expressions.info Alternation page.

Note that RegexOptions.RightToLeft only makes the regex engine examine the input string from right to left, the modifier does not impact how the regex engine processes the pattern itself.

Let me illustrate: if you have a (aaa|bb|a) regex and try to find a match in bbac using Regex.Match, the value you will obtain is bb because a alternative appears after bbb. If you use Regex.Matches, you will get all matches, and both bb and a will land in your results.

Also, the fact that the regex pattern is examined from left to right makes it clear that inside a non-anchored alternative group, the order of alternatives matter. If you use a (a|aa|aaa) regex to match against abbccaa, the first a alternative will be matching each a in the string (see the regex demo). Once you add word boundaries, you can place the alternatives in any order (see one more regex demo).

More Related Contents:

  1. extract number out strings c# [closed]
  2. RegEx to remove key and certificate headers and footers [duplicate]
  3. c# Regular Expression for the text below? [closed]
  4. Get string between two strings in a string
  5. Using RegEx to balance match parenthesis
  6. Using String Format to show decimal up to 2 places or simple integer
  7. How do I remove all non alphanumeric characters from a string except dash?
  8. Find a string between 2 known values
  9. C#, regular expressions : how to parse comma-separated values, where some values might be quoted strings themselves containing commas
  10. Regular expression for decimal number
  11. How to keep the delimiters of Regex.Split?
  12. How to find multiple occurrences with regex groups?
  13. Getting overlapping regex matches in C#
  14. Regex: C# extract text within double quotes
  15. .Net regex matching $ with the end of the string and not of line, even with multiline enabled
  16. How do I filter all HTML tags except a certain whitelist?
  17. Why \b does not match word using .net regex
  18. RegEx for an IP Address
  19. Regex word boundary expressions
  20. Regex to match all us phone number formats
  21. How to remove numbers from string using Regex.Replace?
  22. .Net regex: what is the word character \w?
  23. Canadian postal code validation
  24. Regex to match a word with + (plus) signs
  25. c# regex matches example
  26. How to extract decimal number from string in C#
  27. C# Regex to match the word with dot
  28. C# Regex Issue “unrecognized escape sequence”
  29. Regex for extracting certain part of a String
  30. C# Regex, Unrecognized escape sequence

Leave a Comment