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Search and replace in Ruby - Ruby Regular Expressions

"If you want to replace one part of a string by another in Ruby, you can use the sub method on your string object. The first parameter you give to the method is the string you want to replace, and the second is the string you want to replace it by."

OK - that's the easy bit ... but what if ....

If you want to replace ALL occurrences of a string, you should use the gsub method rather than the sub method

If instead of replacing an exact string, you want to replace something that's a particular pattern you would find within a string, you replace the first parameter with a Ruby style regular expression - there's an example of these [here], including a long block of comments that takes you through each of the elements.

If you want to mention part of the incoming string in the string it is replaced by, you may do so by capturing the sections of interest in round brackets in the incoming regular expression, and referring to them as \1, \2, etc, in the output string

If you want to replace the last match rather than the first, you can't do so directly ... but you CAN start your regular expression with (.*) which will do a "gredy capture" and move whatever's in the rest of the regular expression as far to the right as possible and still match it. So:
  update = message.sub(/(.*)!/,'\1 ;-) ')
will replace the LAST ! with a smiley face ;-)

Finally - you don't always have to use sub and / or gsub. When you do a match or use the =~ operator, the incoming string is matched and split down into three special global variables:
  $` - the bit before the match ("dollar backquote")
  $& - the bit that matched ("dollar ampersand")
  $' - the bit after the match ("dollar forward quote")
which you can then use to rebuild a new string from its elements.

There are examples of all of these on an demonstration from our Ruby Training Courses which I have published on our web site [here]. There is a further example too - showing regular expressions in Ruby for cleaning user input.

(written 2010-01-31, updated 2010-02-01)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Q806 - Regular Expression Cookbook
  [3788] Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [2804] Regular Expression Myths - (2010-06-13)
  [2727] Making a Lua program run more than 10 times faster - (2010-04-16)
  [2702] First and last match with Regular Expressions - (2010-04-02)
  [2563] Efficient debugging of regular expressions - (2010-01-04)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [1840] Validating Credit Card Numbers - (2008-10-14)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [672] Keeping your regular expressions simple - (2006-04-05)

R109 - Ruby - Strings and Regular Expressions
  [4549] Clarrissa-Marybelle - too long to really fit? - (2015-10-23)
  [4505] Regular Expressions for the petrified - in Ruby - (2015-06-03)
  [4388] Global Regular Expression matching in Ruby (using scan) - (2015-01-08)
  [3758] Ruby - standard operators are overloaded. Perl - they are not - (2012-06-09)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [3621] Matching regular expressions, and substitutions, in Ruby - (2012-02-23)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [2980] Ruby - examples of regular expressions, inheritance and polymorphism - (2010-10-02)
  [2623] Object Oriented Ruby - new examples - (2010-02-03)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2614] Neatly formatting results into a table - (2010-02-01)
  [2295] The dog is not in trouble - (2009-07-17)
  [2293] Regular Expressions in Ruby - (2009-07-16)
  [1891] Ruby to access web services - (2008-11-16)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1875] What are exceptions - Python based answer - (2008-11-08)
  [1588] String interpretation in Ruby - (2008-03-21)
  [1195] Regular Express Primer - (2007-05-20)
  [987] Ruby v Perl - interpollating variables - (2006-12-15)
  [986] puts - opposite of chomp in Ruby - (2006-12-15)
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)


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