Hyphen vs Underscore good for SEO URLs

The recurring question in SEO URLs is whether to use a hyphen, or dash, (these-are-hyphens) or an underscore (these_are_underscores) to separate your words in URL’s. Does Search engine recognize them as different?

Though we humans can understand that the both are used in URL’s to separate the words to make it understand better and make it readable. Even though both can server for well for this purpose but there is a bit difference the way the bolts read that.

If you have a C or C++ back ground you may have come across name such as

(TEMP_2)

which means temporary variable 2 , as you may have another temporary variable, with this what you actually mean is temporary variable 2 , which is the same

TEMP2

means but you added under score to make that identifiable easily.

This is what the search engines do they treat underscore as just to separate the words i.e. they treat the whole as a same continuous word.

But if you use hyphen the bolt treat it as different word, this is the difference. That is why hyphen is preferred that underscore.

But don’t change your site URL by rewriting every URL if things are working as it should be. You may have noticed Wikipedia has may URL like Web_design a lot of them are there is Wikipedia but still it has good page rank, I because they have lot of headings and many other things.
If you plan to build a new site prefer hyphen that underscore.