Overview
The author of these articles Are C# .Net Regular Expressions Fast Enough for You? and .Net Regex: Can Regular Expression Parsing be Faster than XmlDocument or Linq to Xml? recently pointed out to me that he had found that Regular expressions were at least as fast or faster than the alternatives in C#. However it has been my experience that regular expressions in Java were slower. It is hard for me to say why this might be different in Java and C# or even if these are fair comparisons but here is what I found in Java.Searching XML for the start of a field
This test searches for the expression "\\s*" in short section of text.Performing 1 loops, regex took 323.389 us and indexOf took 73.026 us on average, ratio=4.4 Performing 10 loops, regex took 61.513 us and indexOf took 42.480 us on average, ratio=1.4 Performing 100 loops, regex took 84.899 us and indexOf took 8.134 us on average, ratio=10.4 Performing 1,000 loops, regex took 16.307 us and indexOf took 5.851 us on average, ratio=2.8 Performing 10,000 loops, regex took 3.751 us and indexOf took 1.321 us on average, ratio=2.8 Performing 100,000 loops, regex took 3.586 us and indexOf took 1.260 us on average, ratio=2.8Using indexOf is somewhat faster, but more complex to code. You would only optimise this example if you really needed to as even one million searches takes a fraction of a second.
Code for Search XML example
Searching for a value= expression in a short string
This test search for the {value} in value="{value}" of a short string.Performing 1 loops, regex took 227.029 us and indexOf took 9.387 us on average, ratio=24.2 Performing 10 loops, regex took 11.141 us and indexOf took 1.195 us on average, ratio=9.3 Performing 100 loops, regex took 9.631 us and indexOf took 1.108 us on average, ratio=8.7 Performing 1,000 loops, regex took 14.993 us and indexOf took 2.491 us on average, ratio=6.0 Performing 10,000 loops, regex took 2.472 us and indexOf took 0.254 us on average, ratio=9.7 Performing 100,000 loops, regex took 0.583 us and indexOf took 0.097 us on average, ratio=6.0 Performing 100,000 loops, regex took 0.465 us and indexOf took 0.032 us on average, ratio=14.5 Performing 100,000 loops, regex took 0.467 us and indexOf took 0.032 us on average, ratio=14.6Since the times keep improving even after 100K iterations, I suspect the JIT is able to optimise indexOf more than the relatively complex code supporting regular expressions.
Code for Searching a short string example
Source : http://vanillajava.blogspot.com/2012/01/regex-vs-indexof-in-java.html
