There is a post which is really hot recently which showcased a best ever regular expression trick. In this post, it provides a trick which can be adapted to situations where one wants to match some patterns desired and exclude the patterns undesired. In simple, the trick is :Pattern not desired | (Pattern desired)Then taking the group 1 match of the capturing group of matched. This group will contain the strings matching the desired patterns.Have to say this trick is really neat and brilliant. Here is a simple explanation on how this works. The above regular expression will first try to m...

When we need to process a HTML page source code, we often need to retrieve the meta description of the page besides the links in the page. This description is usually located in <meta> tag of a HTML page. The meta description is very useful for search engine index. How can we retrieve the meta description? If we use a regular expression, we can easily get the meta description.In JavaScript, the regular expression looks like :var pattern = /<meta.*?name="description".*?content="(.*?)".*?>|<meta.*?content="(.*?)".*?name="description".*?>/i;since the description is the content i...

Rob Pike wrote 30 lines of codes to realize a simple regular expression matcher in his book The practice of Programming. This piece of code is really cool. Let's take a look at the code.Meaning of different characters.CharacterMeaningcGeneral character.Match any single character^Match start of a string$Match end of a string*Match zero or many occurrences of a character/*match ：Test the regexp in text*/int match(char* regexp,char* text){    if(regexp[0] == '^')        return matchhere(regexp+1,text);     do{ /*Check the strin...

Sometimes we may have strings which contain some UR;s and we may want to retrieve the hostname from the URLs for some statistic use. For example, we may have a URL : http://www.example.com/aboutus.html. We may want to retrieve the www.example.com from the URL. How? Use regular expression. Here I give an example using JavaScript. If you want to check whether a string is a URL or not. Refer to Detect URLs in a Block of Text. In JavaScript, we can have a regular expression like var pattern=/(.+:\/\/)?([^\/]+)(\/.*)*/i;the regular expression pattern can be used to get the hostname. There are three...

OverviewThe 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 fieldThis test searches for the exp...

perl -lne '(1x$_) =~ /^1?$|^(11+?)\1+$/ || print "$_ is prime"'Can you figure out how it works? I give an explanation below, but try to figure it out yourself. Here is what happens when you run it:$ perl -lne '(1x$_) =~ /^1?$|^(11+?)\1+$/ || print "$_ is prime"'122 is prime33 is prime455 is prime677 is prime89101111 is primeHere is how it works.First, the number is converted in its unary representation by (1x$_). For example, the number 5 gets converted into 1x5, which is 11111 (1 repeated 5 times.)Next, the unary string gets tested against the regular expression. If it matches, the number is ...