Why is this regular expression so slow?

Question

I am trying to trim leading and trailing whitespace and newlines from a string. The newlines are written as \n (two separate characters, slash and n). In other words, it is a string literal, not a CR LF special character.

For example, this:

\n \nRight after this is a perfectly valid newline:\nAnd here is the second line. \n

Should become this:

Right after this is a perfectly valid newline:\nAnd here is the second line.

I came up with this solution:

text = text
        .replace(/^(\s*(\\n)*)*/, '') // Beginning
        .replace(/(\s*(\\n)*)*$/, '') // End

These patterns match just fine according to RegexPal.

However, the second pattern (matching the end of the string) takes a very long time — about 32 seconds in Chrome on a string with only a couple of paragraphs and a few trailing spaces. The first pattern is quite fast (milliseconds) on the same string.

Here is a CodePen to demonstrate it.

Why is it so slow? Is there a better way to go about this?

Answer 1

The reason it takes so long is because you have a * quantifying two more *

A good explanation can be found in the PHP manual, but I don't think JavaScript supports once-only subpatterns.

I would suggest this regex instead:

text = text.replace(/^(?:\s|\\n)+|(?:\s|\\n)+$/g,"");