Explain this regular expression: (?:|{}I )

Question

I am not new to regular expressions and I use them all the time. Except, I just don't understand this one. Here is the expression in full:

/^(?:|{}I )am on (.+)\$/

I understand everything in this regular expression except the (?:|{}I ) part, and what its relation is in the context of the whole regular expression.

Any help would be much appreciated.

Answer 1

That part matches a subpattern at the very beginning of the string, which can be:

• Either nothing (the part between ?: and | is empty), or

• An opening curly brace { followed by a closing curly brace } followed by the letter I followed by a space character.

The ?: means it doesn't capture, so the first captured subpattern is (.+), not (?:|{}I ).

Typically, the { and } characters are used in regular expressions for quantifying a certain pattern (e.g. \d{0,5} means 0 to 5 digits), but in this case they have no special meaning since there are no digits or commas between them.

In relation to the whole regular expression, I'm guessing that it's supposed to match a string that either starts with "am on...", or "{}I am on...", though I have no idea why the curly braces are needed or why the $ is escaped with a \ at the very end.