What is the use of ? in grep command.. And practical use

Question

I tried to find the actual practical use of ? i.e. for e.g. "egrep a? filename" but was not able to find any.. It returns all results..

So, Please help me out wherein i could know the actual use of egrep ? command..

If i use 'a?', it returns all result i.e. strings or lines. which has 0 a's, 1 a's, 2 a's and so on.. i.e. i am not able to find the use of the same..

Thanks

Answer 1

Say you wanted to match numeric assignment expressions like this in a script:

x=1234

where some numbers are negative and have a minus sign:

x=-5678

You could use this:

grep -E "x=-?[0-9]+" *

The question mark makes the minus optional.

(I don't think plain grep supports ? or +, hence -E).

Answer 2

It's a single character search, matching one or zero of the character before it.

Note: you need to escape the ? by using a \ first: \?.

Answer 3

? is not a metacharacter in basic grep, so if you're grepping for a?, you are in fact grepping for a followed by a question mark.

If you were using egrep (aka grep -E), then answers indicating that ? is a zero-or-one-of-previous-entity regex metacharacter would be correct.

Answer 4

In regex speak, a? means 0 or 1 'a's. So if you search for a string that has 0 or 1 a's in it, you'll get everything. A place where it would be useful is matching positive integers:

/^\+?\d+$/

which plays out as

^: beginning of line
\+: + sign
?: 0 or 1 of previous character
\d: digit
+: one or more of previous character
$: end of line

and would match both +123 and 456

Have a look at regular-expressions.info for more info on using regex's.