What /\b([a-zA-Z]+) \1\b/gi mean in Javascript RegExp

Question

I am studying regexp, there are examples. What's the different of g1 and g2 ?

var g1 = /\b([a-zA-Z]+) \b/gi.exec("Is is this is a the cost of of gas going up?");
var g2 = /\b([a-zA-Z]+) \1\b/gi.exec("Is is this is a the cost of of gas going up?");

console.log(g1);      // result is ["Is ", "Is", index: 0"]
console.log(g2);      // result is ["Is is", "Is", index: 0"]

`\1` is a back reference, that's why `Is` is matched twice in the seconds example. — Josh Crozier, Dec 18 '15 at 02:14
Please refer to: [What does this Regex Mean?](http://stackoverflow.com/q/22937618/1620622) in the future for help on what a certain regex does — Downgoat, Dec 18 '15 at 02:53
If you're using the `i` flag, you don't need to say `[a-zA-Z]`. — , Dec 18 '15 at 05:26

Dima Parzhitsky · Answer 1 · 2015-12-18T15:20:20.843

A bunch of upper- and lowercase letters followed by space and that exact bunch second time. For example: UserName UserName or jnDJkjLKSdcSJ jnDJkjLKSdcSJ.

The difference is that g1 doesn't match the repetition of this bunch, because there is no \1 in it.

Clarified:

/\b([a-zA-Z]+) \b/gi

/       start regexp
\b      match a word boundary
(…)     create a capturing group
[…]     create a class of characters to match one of them
a-z     match any character between "a" and "z" ("a", "b", "c", "d", "e" …)
A-Z     match any character between "A" and "Z" ("A", "B", "C", "D", "E" …)
+       match one or more of a preceding token (see above)
        match space sign (exactly)
\1      match first capturing group
\b      match a word boundary
/gi     finish regexp and set flags "global" and "case-insensitive"

\b([a-zA-Z]+) \b

(picture is clickable)

It would be nice to provide information such as (1) what the `\1` is called (2) and a link to documentation. — , Dec 18 '15 at 05:25

What /\b([a-zA-Z]+) \1\b/gi mean in Javascript RegExp

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