Conditional statement using rexep with bash - regex

Question

I want to use the "conditional regexp" structure from bash, present since the third version of bash (circa 2004).
It's supposed to go like this:
if [[ $string =~ $regexp ]]; then
#do smthg
else
#do somthg else
fi
So here's my code, following this structure, and its role is to check if the name contained in SSID is presend in the output from iw dev wlan0 link :
if [[ $(iw dev wlan0 link) =~ $SSID+ ]]; then
#do sthming
else
echo "wrong network"
fi
For some reason that i can't decipher, this statement works pretty well if I run it right into the bash shell, like
if [[ $(iw dev wlan0 link) =~ $SSID+ ]]; then echo found; else echo not found; fi
But if i run it inside the script its contained, it'll spit out:
scripts/ssidchecker.sh: 22: [[: not found
22 being the line of the "fi" keyword. The strangest thing is that it will always execute the code contained in the "else" statement
Is "not found" meant to indicate me that the regexp dind't find anything in that string? Is it a real error message?

Answer 1

Starting Note: Answer created with input from comments in question, especially in response of the last comment.
First, what is [[? It is a shell keyword.
samveen#maverick:~$ type [[
[[ is a shell keyword
This means that [[ is an internal keyword of bash, not a command and thus will not work with other shells. Thus, your error output
scripts/ssidchecker.sh: 22: [[: not found
means that the shell you're using is probably
Not bash
A version of bash older than 2.02 ([[ introduced in Bash-2.02)
Given that 2.02 is a really really old version (pre Y2K), all this just points to the fact that the shell you're using to run the script is probably not /bin/bash, instead probably being /bin/sh which is the most common path used for Bourne shell scripts shebang (the #!) line.
Please change that to /bin/bash or explicitly run bash scripts/ssidchecker.sh and you're good to go.
As to why the else section was always executed, the [[ command not being found is the same as a failure (non-zero return value), from the viewpoint of if. Thus the else section is executed.
samveen#maverick:~$ /bin/whosyourdaddy
-bash: /bin/whosyourdaddy: No such file or directory
samveen#maverick:~$ echo $?
127
As a side note on portability, the bash hackers wiki also says the following about the [[ keyword:
Amongst the major "POSIX-shell superset languages" (for lack of a
better term) which do have [[, the test expression compound command is
one of the very most portable non-POSIX features. Aside from the =~
operator, almost every major feature is consistent between Ksh88,
Ksh93, mksh, Zsh, and Bash. Ksh93 also adds a large number of unique
pattern matching features not supported by other shells including
support for several different regex dialects, which are invoked using
a different syntax from Bash's =~, though =~ is still supported by ksh
and defaults to ERE.
Thus your script will probably fail on the =~ even if the shell supports [[, in case the shell isn't bash but supports [[.