The closure, or asterisk or star, means that the item immediately preceded by * is matched zero or more times. For example, in the given expression, we match a letter, immediately followed by a lowercase letter or digit. The first character class matches a letter. The second character class matches a letter or digit. The star repeats the second character class:
$ echo -e "ca\nc\nc1\n1\n;\nc;\ncc" | awk '/[a-z][a-z0-9]*/'
The output on execution of this code is as follows:
Let's have another example to explain it. In this example, we print all the lines that contain the ca string and it is followed by zero or more occurrences of t:
$ echo -e "ca\ncat\ncatt\nc\ncatterpillar" | awk '/cat*/'
The output on execution of this code ...