Ok so this lesson is easy but they explain it in a stupid way that makes it difficult.
So imagine you want to match
abc123abc123
you could match it like this…
/(abc)(123)(abc)(123)/
but what is you don’t feel like typing (abc) and (123) twice?
well then you can reference the first times you typed them to save your fingers! Javascript would ‘label’ them like this.
\1 \2 \3 \4 //top line is the labels.
/(abc)(123)(abc)(123)/
so insted I can just write
/(abc)(123)\1\2/
\1 references what is inside the first set of () ie abc
\2 references what is inside the second set of () ie 123
one more example.
If I wanted to match
xyz xyz xyz abc abc xyz abc
I could use
/(xyz)\s\1\s\1\s(abc)\s\2\s\1\s\2
in this example \1 is equal to xyz,
\2 is equal to abc.
Hopwe this helps.