About the syntax of regular expression

Question

I found a piece of code that accidentally outputs strange and inefficient regular expressions such as

a(a|a)b

which gets an a|a (two identical expressions connected by |)

Is such expression syntactically right? I can easily fix it, but I just want to know whether such expressions are valid or not, because this one seems bugs-free so far.

Answer 1

What the regex a(a|a)b indicates is,

And the regex aab indicates

And both will matches the input aab

Answer 2

Yep, this is syntactically equivalent to the regex /a(a)b/ - just slower.