bash regex: asterisk metacharacter kills redundant newline characters

Question

To test the basic functioning of the asterisk metacharacter in search operations using grep, I used a file (regex.txt) with the following contents:

$ cat regex.txt
1
11
111
1111
11111





$

There are 6 newlines after the 11111 in my example, intentionally added.

Question 1.

Why does the output of grep "11*" regex.txt not include the newlines after 11111 unlike the output of grep "1*" regex.txt?

Question 2.

The output of grep "111*" regex.txt is

11
111
1111
11111

The output of grep "1111*" regex.txt is

111
1111
11111

The output of grep "11111*" regex.txt is

1111
11111

Why are the top layers missing for every increase in '1' in the search key?

Answer 1

Your two questions are basically the same. In regular expressions, * means "match the previous character 0 or more times". So, 1* will match 0 or more 1 but 11* will match a 1 followed by 0 or more 1. This means that

• grep "11*" will only print lines containing at least one 1. The * only applies to the second 1, so the first is obligatory. That's why you don't see the empty lines, they don't match 1.

• grep "1*" will match 0 or more 1. Here, you don't have a preceding character that needs to be matched so the blank lines are also matched because they contain 0 1.

• grep "111*", grep "1111*" etc don't match for the same reason as the first point above. 111* will only print lines containing at least 2 1, possibly more. 1111* will match lines containing at least 3 1, possibly more.

The main point here is that the * only affects the character immediately before it, not the entire pattern. To give a quantifier to an entire pattern you can use parentheses:

$ grep '\(111\)*' regex.txt 
1
11
111
1111
11111





$    

Here, the parentheses (they need to be escaped with \ so they won't be included in the search pattern) group the characters together so the * is applied to the group. As you see above, that matches lines containing 0 or more occurrences of 111, so it prints all lines.

Answer 2

'1*' selects zero or more characters that are one. 1 selects one character that is one. A blank line doesn't have have any characters that are one, but it does have zero characters that are one.