正则表达式X和/或Y.

Question

Consider the following regular expression, where X and Y are any regex.

XY|YX|X|Y

This regex tests for X and/or Y , but has to double-check expressions if the "and" clause is not found.

What is the best way to match X and/or Y ?

Answer 1

The regex /XY?|YX?/ should work to match each of the four situations you listed. This is limited however, as if the regex you use have a common match, you may get unexpected results.

With limited information, comes limited solutions

If this is an issue, perhaps add more requirements to the question.

Explanation:

The first half will attempt to match the X regex then will attempt to match the Y regex 0 or 1 times. If the X regex fails, then it will try the second half; which will attempt to match the Y regex then will attempt to match the X regex 0 or 1 times.

Answer 2

[XY]+ to get any number of Xs and Ys (so includes XYYXYX , etc).

Or...

[XY]{1,2} to select 1-2 Xs and Ys (which only includes your 4 examples).

Answer 3

For your requirement of X and/or Y how about this regex:

(?:(?=.*?X)|(?=.*?Y))(X|Y)

Live Demo: http://www.rubular.com/r/K36HjXzHLF