Ruby Programming Language :: Error when substituting a backslash in a string


	Error when substituting a backslash in a string Hi, Why can't I substitute a single backslash in a string? a = "\\6" p a # => "\\6" puts a # => \6 p a.sub('\', '') Expected: "\6" Got: j:4: unterminated string meets end of file j:4: parse error, unexpected $, expecting ')' If I change the replacement to 'c', I get a new error: j:4: parse error, unexpected tIDENTIFIER, expecting ')' puts a.sub('\', 'c') ^ j:4: unterminated string meets end of file Both substitution work fine when the pattern is a regular character. The substitution also works if num('\') is a multiple of 2, example: a = '\\6' p a # => "\\6" puts a # => \6 p a.sub('\\', '') Expected: "6" Got: "6" Thanks for your time. On 6/6/07, Federico Zagarzaz <fzagarz@gmail.com> wrote: > Hi, > Why can't I substitute a single backslash in a string? > a = "\\6" > p a # => "\\6" > puts a # => \6 > p a.sub('\', '') On that last line, your backslash is escaping the single quote and causing parsing lossage. How's about this: p a.sub("\\", '') Hope that helps, -Harold On Jun 6, 2:59 pm, "Federico Zagarzaz" <fzagarz@gmail.com> wrote: > Why can't I substitute a single backslash in a string? > a = "\\6" > ... > p a.sub('\', '') Because you're essentially escaping a single quote in your String#sub method call. Double escape that too: irb(main):001:0> a = "\\6" => "\\6" irb(main):002:0> a.sub('\\', '') => "6" Why is this? Suppose I wanted to make a string with single quotes in it: irb(main):003:0> a = '\'hello\'' => "'hello'" Thanks for your answers, silly me, got confused :), But, how does it work?, suppouse you want to convert "fede" to "f\4de" 1) puts 'fede'.sub('e', '\4') # => fde 2) puts 'fede'.sub('e', '\\4') # => fde 3) puts 'fede'.sub('e', '\\\4') # => f\4de 4) puts 'fede'.sub('e', '\\\\4') # => f\4de 5) puts 'fede'.sub('e', '\\\\\4') # => f\de 6) puts 'fede'.sub('e', '\\\\\\4') # => f\de 7) puts 'fede'.sub('e', '\\\\\\\4') # => f\\4de . I don't get it.. I was thinking that it might work by making two passes looking for and converting backslashes, like this: (7): first pass : (\\)(\\)(\\)(\4) \ \ \ \4 second pass: (\\)(\\)4 result : \\4 But I'm not 100% sure, do you know how it works? Thanks again. On 6/6/07, james.d.mast@gmail.com <james.d.mast@gmail.com> wrote: - Hide quoted text - > On Jun 6, 2:59 pm, "Federico Zagarzaz" <fzagarz@gmail.com> wrote: > > Why can't I substitute a single backslash in a string? > > a = "\\6" > > ... > > p a.sub('\', '') > Because you're essentially escaping a single quote in your String#sub > method call. Double escape that too: > irb(main):001:0> a = "\\6" > => "\\6" > irb(main):002:0> a.sub('\\', '') > => "6" > Why is this? Suppose I wanted to make a string with single quotes in > it: > irb(main):003:0> a = '\'hello\'' > => "'hello'" >> p a.sub('\', '') > On that last line, your backslash is escaping the single quote and > causing parsing lossage. so.. sometimes escaping happens within single quotes, and sometimes it doesn't? a = 'yo\n' # doesn't escape feels like this should escape: p a.sub("\\", '') and this shouldn't: p a.sub('\\', '') one more thing to remember I guess.. :-) -- Posted via http://www.ruby-forum.com/. On Jun 6, 4:27 pm, "Federico Zagarzaz" <fzagarz@gmail.com> wrote: > 1) puts 'fede'.sub('e', '\4') # => fde > 2) puts 'fede'.sub('e', '\\4') # => fde > 3) puts 'fede'.sub('e', '\\\4') # => f\4de > 4) puts 'fede'.sub('e', '\\\\4') # => f\4de > 5) puts 'fede'.sub('e', '\\\\\4') # => f\de > 6) puts 'fede'.sub('e', '\\\\\\4') # => f\de > 7) puts 'fede'.sub('e', '\\\\\\\4') # => f\\4de You have to be careful with using backslashes with a number for a replacement string. In a replacement string, \# (where # is a number) indicates that you want to insert something that was matched in the original string - the matched things are in parentheses and occur in regular expressions. For example: irb(main):001:0> a = "Page 3 of 86" => "Page 3 of 86" irb(main):002:0> a.sub(/^Page (\d+) of (\d+)$/, 'You are on page \1 out of a total of \2 pages') => "You are on page 3 out of a total of 86 pages" The PickAxe has an excellent subject on this and goes over your question with all sorts of backslash permutations. I guarantee that it will clarify things further for you: http://www.rubycentral.com/book/tut_stdtypes.html (see "Backslash Sequences in the Substitution" section) Thanks for the link, it cleared my doubts. On 6/7/07, james.d.mast@gmail.com <james.d.mast@gmail.com> wrote: - Hide quoted text - > On Jun 6, 4:27 pm, "Federico Zagarzaz" <fzagarz@gmail.com> wrote: > > 1) puts 'fede'.sub('e', '\4') # => fde > > 2) puts 'fede'.sub('e', '\\4') # => fde > > 3) puts 'fede'.sub('e', '\\\4') # => f\4de > > 4) puts 'fede'.sub('e', '\\\\4') # => f\4de > > 5) puts 'fede'.sub('e', '\\\\\4') # => f\de > > 6) puts 'fede'.sub('e', '\\\\\\4') # => f\de > > 7) puts 'fede'.sub('e', '\\\\\\\4') # => f\\4de > You have to be careful with using backslashes with a number for a > replacement string. In a replacement string, \# (where # is a number) > indicates that you want to insert something that was matched in the > original string - the matched things are in parentheses and occur in > regular expressions. For example: > irb(main):001:0> a = "Page 3 of 86" > => "Page 3 of 86" > irb(main):002:0> a.sub(/^Page (\d+) of (\d+)$/, 'You are on page \1 > out of a total of \2 pages') > => "You are on page 3 out of a total of 86 pages" > The PickAxe has an excellent subject on this and goes over your > question with all sorts of backslash permutations. I guarantee that > it will clarify things further for you: > http://www.rubycentral.com/book/tut_stdtypes.html > (see "Backslash Sequences in the Substitution" section) From: http://www.rubycentral.com/book/tut_stdtypes.html "Within single-quoted strings, two consecutive backslashes are replaced by a single backslash, and a backslash followed by a single quote becomes a single quote. Double-quoted strings support a boatload more escape sequences." easy enough :) -- Posted via http://www.ruby-forum.com/.