On the use of OR | operator in regular expression

I had some difficulties to use the OR | operator in regular expressions.
Let do some exercises and examine the results:
gregexpr("bc", "abcd") : return 2; normal, bc is located at the second position
gregexpr("b|c", "abcd") : return 2 3; | is the OR operator and it will return the position of b OR c
this is the same than :
gregexpr("[bc]", "abcd") or gregexpr("[b|c]", "abcd")

But | and [] are not always the same:

Let do more complicated:
gregexpr("ab|cd", "abcd")
My problem was about the priority order of | operator. The return here is 1 3
It means that the search was for a followed by b OR c followed by d.

Let do more complicated :
gregexpr("a[bc]d", "abcd") : return -1; it search for abd or acd. None exist

In practice, it is good to remember that | separates blocks of comparisons.

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