How does this glob expression remove colons?

Question

Say we have a file called "Hello World: The easiest program" and its name is saved in a variable called "title".

I want to match this with glob pattern matching, but remove the colon. In other words, I want it to match "Hello World The easiest program"

I found out you can do this by using the following:

${/:/|${title}|}

It works, but I don't understand why.

I know that "|" is OR, so to me it looks like it matches "${/:/}" or "${${title}}" or "${}", none of which look like they're removing the colon.

So my question is: How/why does the glob pattern matching expression above remove colons from a title?

Edit: I should mention the shell I'm using is the Steam ROM Manager. And it seems I was mistaken and it's not a glob expression, but a Steam ROM Parser function variable that optionally uses glob syntax (in place of the default regex syntax).Thanks Steeldiver and ilkkachu!

what shell are you using? It seems that ${/:/|${title}|} doesn't work for my current shells (bash and zsh) — Edgar Magallon
– Edgar Magallon, Commented Nov 5, 2022 at 21:45
I am using the Steam ROM Manager on Steam Deck. I didn't understand that the thing between the forward slashes will be replaced. — Rudi Österreicher
– Rudi Österreicher, Commented Nov 6, 2022 at 9:38
Parser Type in Steam ROM Manager was set to Glob. With Hack Saws help, I was now able to use this knowledge to remove brackets, parentheses, their contents and any whitespaces before or after using: ${/(\s?\[.*?\]\s?)/|${/(\s?$.*?$\s?)/|${/:/|${title}|}|} It first takes the title, replaces colons with nothing, then takes the result and replaces the first thing that matches \s?$.*?$\s? and replaces it with nothing and then takes the result again and replaces the first thing that matches \s?\[.*?\]\s? and replaces it with nothing — Rudi Österreicher
– Rudi Österreicher, Commented Nov 6, 2022 at 9:46
The Steam ROM Manager seems hardly a common Unix-y shell, so in the least you really should edit your question to include that context. — ilkkachu
– ilkkachu, Commented Nov 6, 2022 at 17:32

steeldriver · Accepted Answer · 2022-11-06 14:26:12Z

The expression in your question is not a glob expression, it's apparently a Steam ROM Parser function variable that optionally uses glob syntax (in place of the default regex syntax) for its pattern matching sequence.

The expression is of the form

${/pattern/|input|}

with pattern equal to : and input equal to variable expansion ${title}. In this context | is to be read as a kind of ternary conditional

|on match|no match(optional)

So ${/:/|${title}|} means to match occurences of : in ${title} and replace them with nothing.

Note that since : is not special in either shell glob or regular expression syntax, it shouldn't actually matter whether you set the Parser Type to glob or otherwise.

Hack Saw · Accepted Answer · 2022-11-05 22:26:46Z

In bash, assuming you have title="Hello World: The easiest program", if you use Parameter Expansion like this:

echo ${title/:/}

It will print out

Hello World The easiest program

${title/:/} says "take the parameter title, but the first time there is a ':', sub in a ''." (That is, nothing)

If you use an extra /, like this: ${title//:/} it will substitute all of them.

title="Hello World: The easiest:::::: program"
echo ${title//:/}
Hello World The easiest program

With an obvious substitution:

title="Hello World: The easiest:::::: program"
echo ${title//:/*}
Hello World* The easiest****** program

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