Why does the order of alternatives matter in regex?
.net, c#, regex
Solution
The regular expression engine tries to match the alternatives in the order in which they are specified. So when the pattern is `(foo|foobar)&?` it matches `foo` immediately and continues trying to find matches. The next bit of the input string is `bar& b` which cannot be matched.
In other words, because `foo` is part of `foobar`, there is no way `(foo|foobar)` will ever match `foobar`, since it will always match `foo` first.
Occasionally, this can be a very useful trick, actually. The pattern `(o|a|(\w))` will allow you to capture `\w` and `a` or `o` differently:
Regex.Replace("a foobar& b", "(o|a|(\\w))", "$2") // fbr& b
Problem
Code ``` using System; using System.Text.RegularExpressions; namespace RegexNoMatch { class Program { static void Main () { string input = "a foobar& b"; string regex1 = "(foobar|foo)&?"; string regex2 = "(foo|foobar)&?"; string replace = "$1"; Console.WriteLine(Regex.Replace(input, regex1, replace)); Console.WriteLine(Regex.Replace(input, regex2, replace)); Console.ReadKey(); } } } ``` Expected output ``` a foobar b a foobar b ``` Actual output ``` a foobar b a foobar& b ``` Question Why does replacing not work when the order of "foo" and "foobar" in regex pattern is changed? How to fix this?