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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
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
R109 - Ruby - Strings and Regular Expressions
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)
  [986] puts - opposite of chomp in Ruby - (2006-12-15)
  [987] Ruby v Perl - interpollating variables - (2006-12-15)
  [1195] Regular Express Primer - (2007-05-20)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1588] String interpretation in Ruby - (2008-03-21)
  [1875] What are exceptions - Python based answer - (2008-11-08)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1891] Ruby to access web services - (2008-11-16)
  [2293] Regular Expressions in Ruby - (2009-07-16)
  [2295] The dog is not in trouble - (2009-07-17)
  [2614] Neatly formatting results into a table - (2010-02-01)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2623] Object Oriented Ruby - new examples - (2010-02-03)
  [2980] Ruby - examples of regular expressions, inheritance and polymorphism - (2010-10-02)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [3621] Matching regular expressions, and substitutions, in Ruby - (2012-02-23)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [3758] Ruby - standard operators are overloaded. Perl - they are not - (2012-06-09)
  [4388] Global Regular Expression matching in Ruby (using scan) - (2015-01-08)
  [4505] Regular Expressions for the petrified - in Ruby - (2015-06-03)
  [4549] Clarrissa-Marybelle - too long to really fit? - (2015-10-23)

Q806 - Regular Expression Cookbook
  [672] Keeping your regular expressions simple - (2006-04-05)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1840] Validating Credit Card Numbers - (2008-10-14)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [2563] Efficient debugging of regular expressions - (2010-01-04)
  [2702] First and last match with Regular Expressions - (2010-04-02)
  [2727] Making a Lua program run more than 10 times faster - (2010-04-16)
  [2804] Regular Expression Myths - (2010-06-13)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [3788] Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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