Remove leading whitespaces using variable length lookbehind in RegExp

Question

Hello, I'm wondering if variable length lookbehind assertions are supported in JavaScript's RegExp engine?

For example, I'm trying to match the string "variable length" in the string "[a lot of whitespaces and/or tabs]variable length lookbehind", and I have something like this but it does not go well in various RegExp testers:

^(?<=[ \t]+).+(?= lookbehind)

If it's an illegal pattern, what would be a good workaround to it? Thanks!

Answer 1

Javascript doesn't have look-behind at all. Steven Levithan has written up a few says to sort of mimic it, which may be helpful.

I don't quite understand your example, because it seems as though this would fit the bill:

/^\s+(.+)lookbehind$/

...which matches one or more whitespace chars followed by one or more of any character (in a capture group) followed by the word "lookbehind". Used like this:

var str = "        variable length lookbehind";
var match = /^\s+(.+)lookbehind$/.exec(str);

yeilds this array:

match[0]: |        variable length lookbehind|
match[1]: |variable length|

In Javascript, the first entry in the array is the entire matched string, and the subsequent entries are the capture groups.

But you clearly have a good grasp of regex, so I'm not sure that's what you're looking for...

Something to be aware of in this general area is that a number of implementations of RegExp engines in Javascript don't quite handle \s correctly (they miss out matching some whitespace chars above the ASCII range); see the S_REGEXP_WHITESPACE_CHARACTER_CLASS_BUGGY test here.

Answer 2

Javascript regex engine does not support lookbehinds, only lookaheads are supported. Here you can find a mimicking solution: http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript

Answer 3

See Faster JavaScript Trim

It shows numerous ways a trim can be done (none of which require a look-behind) and also compares the speed of the different approaches. If the end-goal includes stripping leading white-space, don't be afraid to break it down into multiple operations.

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems - Jamie Zawinski

Answer 4

I don't know if this would help you with RegExp but if you want to remove whitespaces you can use a trim function

function trimAll(sString) {
  while (sString.substring(0, 1) == ' ') {
    sString = sString.substring(1, sString.length);
  }
  while (sString.substring(sString.length - 1, sString.length) == ' ') {
    sString = sString.substring(0, sString.length - 1);
  }
  return sString;
}

Otherwise if you want to check string existance you can use indexOf IndexOf on StackOverflow.com