Precedence in regular expressions

In regular expressions, the '|' alternation operator has the lowest precedence, then concatenation, followed by the +, *, and ? repetition operations as well as brackets, '{' and '}'. As in arithmetic expressions, operators of higher precedence are done before lower ones. Also, parentheses can change how operators are grouped. Due to these conventions, parentheses are often omitted, as follows:

$ awk '/Jane|Emily/' emp.dat is same as
$ awk '/(Jane)|(Emily)/' emp.dat

The output on execution of the preceding code is as follows:

Jane    Kaur    9837432312  [email protected]      F   hr      1800
Emily   Kaur    8826175812  [email protected]     F   Ops     2100

In POSIX, AWK, and GAWK, the '*', '+', and '?' operators stand for themselves when there is nothing in the regular expression that precedes them. For example, /+/ matches a literal plus sign:

$ echo -e "This is new line
one + one is two
posix regular expression" | awk '/+/{ print }'

The output on execution of the...