Bash need to test for alphanumeric string
I'd invert the logic. Test for invalid characters and echo a warning if at least one is present:
if [[ "$TITLE" =~ [^a-zA-Z0-9] ]]; then echo "INVALID"fiWith that said, your original check worked for me, so you probably need to provide more context (i.e. a larger portion of your script).
why cant we use alnum
[[ 'mystring123' =~ [:alnum:] ]] && echo "ok" || echo "no"
the nominated answer is wrong. Because it doesn't check to the end of the string. also it's inverted. as the conditional says: "if the start of the string is valid characters then echo invalid"
[[ $TITLE =~ ^[a-zA-Z0-9_-]{3,20}$ ]] && ret="VALID" || ret="INVALID"echo $ret