Question

I wonder the general rule to use regular expression in if clause in bash?

Here is an example

$ gg=svm-grid-ch  
$ if [[ $gg == *grid* ]] ; then echo $gg; fi  
svm-grid-ch  
$ if [[ $gg == ^....grid* ]] ; then echo $gg; fi  
$ if [[ $gg == ....grid* ]] ; then echo $gg; fi  
$ if [[ $gg == s...grid* ]] ; then echo $gg; fi  
$   

Why the last three fails to match?

Hope you could give as many general rules as possible, not just for this example.

Answer 1

When using a glob pattern, a question mark represents a single character and an asterisk represents a sequence of zero or more characters:

if [[ $gg == ????grid* ]] ; then echo $gg; fi

When using a regular expression, a dot represents a single character and an asterisk represents zero or more of the preceding character. So “.*” represents zero or more of any character, “a*” represents zero or more “a”, “[0-9]*” represents zero or more digits. Another useful one (among many) is the plus sign which represents one or more of the preceding character. So “[a-z]+” represents one or more lowercase alpha character (in the C locale – and some others).

if [[ $gg =~ ^....grid.*$ ]] ; then echo $gg; fi