regex


About the syntax of regular expression


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.
Yep, this is syntactically equivalent to the regex /a(a)b/ - just slower.
What the regex a(a|a)b indicates is,
And the regex aab indicates
And both will matches the input aab

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