Testing Regular Expressions

Please show exactly how you used that. Cut-and-paste if possible.

Hi Glen. I was just testing it as I input it above. e.g. I tested ^/([0-9])|([0-9])$ and I get a different error "substitution failed". So I am just trying to understand the core of it. I have read various articles about expressions but need help in understanding.

So you're just typing that into a bash prompt? What do you expect to happen? In bash, you can only use a regular expression as part of a `[[ $var =~ regex ]]` conditional command. See [3.2.5.2 Conditional Constructs](https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs)

Shell syntax is extremely context-dependent; bash will only treat something as a regular expression if it occurs in a place where bash expects to see a regular expression (i.e. in `[[ $var =~ regex ]]`). Since you used that in a general command context, bash saw the `^` and interpreted it as a command history substitution (i.e. it thinks you're telling it to recall and re-use some part of a previous command), because that's what `^` means in a general command context. But it couldn't find a matching previous command ("event"), so you get a "event not found" error.

@glennjackman thank you that is the answer I needed.

@GordonDavisson Thank you for the breakdown of the matter. I understand it better now.

You can use https://regexr.com to play with regular expressions.

I sit in a Tesla and translated this thread with Ai: