Regex Odd/Even Amount

Question

I have a regex question that I do not know how to do. It has to match all strings consisting any number of a's at the beginning and then either a single 0 if the number of a's was even or a single 1 if the number of a's was odd.

How can you keep track of even/odd?

Sample

• aaa1
• aaaa0

Answer 1

^(a(aa)*1|(aa)+0)$

or

^(?:a(?:aa)*1|(?:aa)+0)$ if you are using captures.

The first portion: a(aa)*1 will match any odd number of a's followed by a one, and the second portion: (aa)+0 will match any even number of a's followed by a zero.

You can't keep track of the number of matches of a component of the pattern in regular expressions. They don't have memory. Fortunately, you can get around that limitation in this case.

Answer 2

You could use:

^(?:aa)*(?:a1|0)$