Grep and the % percentage symbol

Question

When using grep on the command line I am getting different results when I use a % percent symbol.

I expect this expression to match it returns nothing:

bash-4.1$ echo ' (in=Gen)' | grep -in '*(*)'

However if I include a leading percentage character it's a match:

bash-4.1$ echo ' (in=Gen)' | grep -in '%*(*)'
1: (in=Gen)

Is the percent % symbol a special or control character in grep? Looking at the available documentation I don't see any reference to it.

Answer 1

If you put any other (valid) character, there would be a match too eg using , :

% echo ' (in=Gen)' | grep -in ',*(*)'
1: (in=Gen)

The problem is with your pattern, *(*) , here:

• * means to match zero or more of the preceding token, as there is no preceding token it matches * literally. As there is no * present, the pattern does not match anything

While in %*(*) :

• %* means to match % zero or more times, as there is no % present, grep matches nothing for %* ; in this case grep only matches the last )

Answer 2

In BRE (Basic regular expressions) that grep uses (as opposed to ERE, extended REs from egrep or grep -E , if a * is nor a quantifier of a preceding expression, it will match the literal star. (This also happens with a ^ that is not at the beginning of an expression, and a $ that is not in a position to match end of line: They are interpreted as the normal characters ^ and $ .

From man 7 regex :

'^' is an ordinary character except at the beginning of the RE or(!) the beginning of a parenthesized subexpression, '$' is an ordinary character except at the end of the RE or(!) the end of a parenthesized subexpression, and '*' is an ordinary character if it appears at the beginning of the RE or the beginning of a parenthesized subexpression (after a possible leading '^').

As to your second example, what is only matched is the closing bracket (marked with a ^ on top):

              ^
echo ' (in=Gen)' | grep --color -in '%*(*)' 
1: (in=Gen)

Notice the use of --color to grep to highlight what actually is matched, this has been pretty invaluable to understand grep's behavior).

The seach you are looking for, is likely this one:

% echo '*(in=Gen)' | grep -in --color '*(.*)'
1:*(in=Gen)