Differences between Javascript regexp literal and constructor

Question

We recently had a bug, after a fellow developer changed a RegExp literal into a constructor call, and I was wondering why there is any difference at all. The exact code was

var parts = new RegExp("/rt:([^@]+)@(\d+)/").exec(tag);

vs the original of

var parts = /rt:([^@]+)@(\d+)/.exec(tag);

When tag is, for example rt:60C1C036-42FA-4073-B10B-1969BD2358FB@00000000077, the first (buggy) call returns null, while the second one returns["rt:60C1C036-42FA-4073-B10B-1969BD2358FB@00000000077", "60C1C036-42FA-4073-B10B-1969BD2358FB", "00000000077"]

Needless to say, I reverted the change, but I'd like to know why is there such a difference in the first place.

Felix Kling · Accepted Answer · 2011-11-01 10:51:55Z

43

There are two problems:

The / are not part of the expression. They are delimiters, marking a regex literal. They have to be removed if you use RegExp, otherwise they match a slash literally.

Secondly, the backslash is the escape character in string literals. To create a literal \ for the expression, you have to escape it in the string.

Thus, the equivalent would be:

new RegExp("rt:([^@]+)@(\\d+)")

Especially the escaping makes expression a bit more difficult to write if you want to use RegExp. It is actually only needed if you want to create expression dynamically, that is, if you want to include text stored in a variable for example. If you have a fixed expression, a literal /.../ is easier to write and more concise.

edited Nov 1, 2011 at 10:51

answered Nov 1, 2011 at 10:09

Felix Kling

820k181 gold badges1.1k silver badges1.2k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

El Ronnoco Over a year ago

Good answer, think I'll delete mine! What if you need to reuse a particular RegExp - is that not a good time to use a constructor? Or would you just say var resuasableRegex = /..pattern../; ?

Felix Kling Over a year ago

@ElRonnoco: Exactly, you can assign a regex literal to a variable as well.

Noam Gal Over a year ago

Thanks for the info. Next time I'll look into the strings better, to make sure there are no escape characters needed. Anyway, literal is much easier for sure.

Daniel Over a year ago

For small regex, literals are definitely the way to go. If you have a dynamic expression or a very long regex to maintain, RegExp is a nice option. Escaping isn't too bad: just "double the [literal] backslashes and escape the quotes" Javascript - The Good Parts, p. 71. Here's code that will get just the quotes: function escape(text) { return text.replace(/['"]/g, "\\$&"); };

Narendra Yadala · Accepted Answer · 2011-11-01 10:09:15Z

4

\d needs to be escaped when passesd to new RegExp constructor. So, it needs to be

var parts = new RegExp("rt:([^@]+)@(\\d+)").exec(tag);

answered Nov 1, 2011 at 10:09

Narendra Yadala

9,6641 gold badge31 silver badges44 bronze badges

Collectives™ on Stack Overflow

Differences between Javascript regexp literal and constructor

2 Answers 2

4 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related