查找启动字符串的制表符数量的最佳/最快方法是什么？

Question

I want to find the number of tabs at the beginning of a string (and of course I want it to be fast running code ;) ). This is my idea, but not sure if this is the best/fastest choice:

//The regular expression
var findBegTabs = /(^\t+)/g;

//This string has 3 tabs and 2 spaces: "<tab><tab><space>something<space><tab>"
var str = "      something  ";

//Look for the tabs at the beginning
var match = reg.exec( str );

//We found...
var numOfTabs = ( match ) ? match[ 0 ].length : 0;

Another possibility is to use a loop and charAt:

//This string has 3 tabs and 2 spaces: "<tab><tab><space>something<space><tab>"
var str = "      something  ";

var numOfTabs = 0;
var start = 0;

//Loop and count number of tabs at beg
while ( str.charAt( start++ ) == "\t" ) numOfTabs++;

Answer 1

In general if you can calculate the data by simply iterating through the string and doing a character check at every index, this will be faster than a regex which much build up a more complex searching engine. I encourage you to profile this but I think you'll find the straight search is faster.

Note: Your search should use === instead of == here as you don't need to introduce conversions in the equality check

function numberOfTabs(text) {
  var count = 0;
  var index = 0;
  while (text.charAt(index++) === "\t") {
    count++;
  }
  return count;
}

Answer 2

Try using a profiler (such as jsPerf or one of the many available backend profilers ) to create and run benchmarks on your target systems (the browsers and/or interpreters you plan to support for your software).

It's useful to reason about which solution will perform best based on your expected data and target system(s); however, you may sometimes be surprised by which solution actually performs fastest, especially with regard to big-oh analysis and typical data sets.

In your specific case, iterating over characters in the string will likely be faster than regular expression operations.

Answer 3

One-liner (if you find smallest is best):

"\t\tsomething".split(/[^\t]/)[0].length;

ie splitting by all non-tab characters, then fetching the first element and obtaining its length.