• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Tim Cooke
  • Campbell Ritchie
  • Paul Clapham
  • Ron McLeod
  • Liutauras Vilda
Sheriffs:
  • Jeanne Boyarsky
  • Rob Spoor
  • Bear Bibeault
Saloon Keepers:
  • Jesse Silverman
  • Tim Moores
  • Stephan van Hulst
  • Tim Holloway
  • Carey Brown
Bartenders:
  • Piet Souris
  • Al Hobbs
  • salvin francis

When "\'something'" or "\\'something'" will not compile

 
Ranch Hand
Posts: 111
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Hi,
I am quite confused on when to use '\' ,'\\' or even '\\\'.

I triple checked on K&B but I still fail 99% of questions regarding compilation of regex (questions about the Scanner class for example).

If someone has time, could try to clarify me these concepts?

From what I understood:

Java has reserved escape sequences :
\n = new line
\b = backslash
\t = tab

al the reserved escape sequence use only 1 '\', so trying to compile '\d' will give a compilation error.

Apart from this all the rest is completely unclear to me.


Thanks all.
 
Sheriff
Posts: 9691
42
Android Google Web Toolkit Hibernate IntelliJ IDE Spring Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Well as you already know java uses \ for escape sequences. If you want to use \ in a String when the escape sequence is not legal, you use \\ instead of \. So in regex when you want to use \d, you'll have to escape \ so you'll write \\d. By doing this you'll pass the compilation and the regex engine will process it as \d...
 
Ranch Hand
Posts: 142
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
One notable exception to the rule of using \\ in the regex pattern is if you pass the regex pattern as a command line argument. If you do so, you simply use \ and not \\. I learnt this the hard way. Here is my made up example code:




Here is how you could run the code after compilation. If you see below, I am just using \ and not \\ and it still works. But this is the only exception to the rule i.e. if you use the regex from the command line. If you hard code the regex inside the code itself, then you need to use \\.
java Test "\w+"
java Test "\d"

But apparently iit also works if you use \\ instead of \ like shown below:
java Test "\\w+"
java Test "\\d"

Trying writing your own code and experimenting with it! Good luck.

 
Ranch Hand
Posts: 62
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
But, if we want to match a \ (backslash):
 
Don't get me started about those stupid light bulbs.
reply
    Bookmark Topic Watch Topic
  • New Topic