[regex] use regular expression in if-condition in bash

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.

Adding this solution with grep and basic sh builtins for those interested in a more portable solution (independent of bash version; also works with plain old sh, on non-Linux platforms etc.)

# GLOB matching
gg=svm-grid-ch    
case "$gg" in
   *grid*) echo $gg ;;
esac

# REGEXP    
if echo "$gg" | grep '^....grid*' >/dev/null ; then echo $gg ; fi    
if echo "$gg" | grep '....grid*' >/dev/null ; then echo $gg ; fi    
if echo "$gg" | grep 's...grid*' >/dev/null ; then echo $gg ; fi    

# Extended REGEXP
if echo "$gg" | egrep '(^....grid*|....grid*|s...grid*)' >/dev/null ; then
  echo $gg
fi    

Some grep incarnations also support the -q (quiet) option as an alternative to redirecting to /dev/null, but the redirect is again the most portable.

Use =~

for regular expression check Regular Expressions Tutorial Table of Contents

@OP,

Is glob pettern not only used for file names?

No, "glob" pattern is not only used for file names. you an use it to compare strings as well. In your examples, you can use case/esac to look for strings patterns.

 gg=svm-grid-ch 
 # looking for the word "grid" in the string $gg
 case "$gg" in
    *grid* ) echo "found";;
 esac

 # [[ $gg =~ ^....grid* ]]
 case "$gg" in ????grid*) echo "found";; esac 

 # [[ $gg =~ s...grid* ]]
 case "$gg" in s???grid*) echo "found";; esac

In bash, when to use glob pattern and when to use regular expression? Thanks!

Regex are more versatile and "convenient" than "glob patterns", however unless you are doing complex tasks that "globbing/extended globbing" cannot provide easily, then there's no need to use regex. Regex are not supported for version of bash <3.2 (as dennis mentioned), but you can still use extended globbing (by setting extglob ). for extended globbing, see here and some simple examples here.

Update for OP: Example to find files that start with 2 characters (the dots "." means 1 char) followed by "g" using regex

eg output

$ shopt -s dotglob
$ ls -1 *
abg
degree
..g

$ for file in *; do [[ $file =~ "..g" ]] && echo $file ; done
abg
degree
..g

In the above, the files are matched because their names contain 2 characters followed by "g". (ie ..g).

The equivalent with globbing will be something like this: (look at reference for meaning of ? and * )

$ for file in ??g*; do echo $file; done
abg
degree
..g

if [[ $gg =~ ^....grid.* ]]