How to remove strings that end with a particular character in Ruby - ruby

Based on "How to Delete Strings that Start with Certain Characters in Ruby", I know that the way to remove a string that starts with the character "#" is:
email = email.gsub( /(?:\s|^)#.*/ , "") #removes strings that start with "#"
I want to also remove strings that end in ".". Inspired by "Difference between \A \z and ^ $ in Ruby regular expressions" I came up with:
email = email.gsub( /(?:\s|$).*\./ , "")
Basically I used gsub to remove the dollar sign for the carrot and reversed the order of the part after the closing parentheses (making sure to escape the period). However, it is not doing the trick.
An example I'd like to match and remove is:
"a8&23q2aas."

You were so close.
email = email.gsub( /.*\.\s*$/ , "")
The difference lies in the fact that you didn't consider the relationship between string of reference and the regex tokens that describe the condition you wish to trigger. Here, you are trying to find a period (\.) which is followed only by whitespace (\s) or the end of the line ($). I would read the regex above as "Any characters of any length followed by a period, followed by any amount of whitespace, followed by the end of the line."
As commenters pointed out, though, there's a simpler way: String#end_with?.

I'd use:
words = %w[#a day in the life.]
# => ["#a", "day", "in", "the", "life."]
words.reject { |w| w.start_with?('#') || w.end_with?('.') }
# => ["day", "in", "the"]
Using a regex is overkill for this if you're only concerned with the starting or ending character, and, in fact, regular expressions will slow your code in comparison with using the built-in methods.
I would really like to stick to using gsub....
gsub is the wrong way to remove an element from an array. It could be used to turn the string into an empty string, but that won't remove that element from the array.

def replace_suffix(str,suffix)
str.end_with?(suffix)? str[0, str.length - suffix.length] : str
end

Related

Convert string to camel case in Ruby

Working on a Ruby challenge to convert dash/underscore delimited words into camel casing. The first word within the output should be capitalized only if the original word was capitalized (known as Upper Camel Case).
My solution so far..:
def to_camel_case(str)
str.split('_,-').collect.camelize(:lower).join
end
However .camelize(:lower) is a rails method I believe and doesn't work with Ruby. Is there an alternative method, equally as simplistic? I can't seem to find one. Or do I need to approach the challenge from a completely different angle?
main.rb:4:in `to_camel_case': undefined method `camelize' for #<Enumerator: []:collect> (NoMethodError)
from main.rb:7:in `<main>'
I assume that:
Each "word" is made up of one or more "parts".
Each part is made of up characters other than spaces, hypens and underscores.
The first character of each part is a letter.
Each successive pair of parts is separated by a hyphen or underscore.
It is desired to return a string obtained by modifying each part and removing the hypen or underscore that separates each successive pair of parts.
For each part all letters but the first are to be converted to lowercase.
All characters in each part of a word that are not letters are to remain unchanged.
The first letter of the first part is to remain unchanged.
The first letter of each part other than the first is to be capitalized (if not already capitalized).
Words are separated by spaces.
It this describes the problem correctly the following method could be used.
R = /(?:(?<=^| )|[_-])[A-Za-z][^ _-]*/
def to_camel_case(str)
str.gsub(R) do |s|
c1 = s[0]
case c1
when /[A-Za-z]/
c1 + s[1..-1].downcase
else
s[1].upcase + s[2..-1].downcase
end
end
end
to_camel_case "Little Miss-muffet sat_on_HE$R Tuffett eating-her_cURDS And_whey"
# => "Little MissMuffet satOnHe$r Tuffett eatingHerCurds AndWhey"
The regular expression is can be written in free-spacing mode to make it self-documenting.
R = /
(?: # begin non-capture group
(?<=^| ) # use a positive lookbehind to assert that the next character
# is preceded by the beginning of the string or a space
| # or
[_-] # match '_' or '-'
) # end non-capture group
[A-Za-z] # match a letter
[^ _-]* # match 0+ characters other than ' ', '_' and '-'
/x # free-spacing regex definition mode
Most Rails methods can be added into basic Ruby projects without having to pull in the whole Rails source.
The trick is to figure out the minimum amount of files to require in order to define the method you need. If we go to APIDock, we can see that camelize is defined in active_support/inflector/methods.rb.
Therefore active_support/inflector seems like a good candidate to try. Let's test it:
irb(main)> require 'active_support/inflector'
=> true
irb(main)> 'foo_bar'.camelize
=> "FooBar"
Seems to work. Note that this assumes you already ran gem install activesupport earlier. If not, then do it first (or add it to your Gemfile).
In pure Ruby, no Rails, given str = 'my-var_name' you could do:
delimiters = Regexp.union(['-', '_'])
str.split(delimiters).then { |first, *rest| [first, rest.map(&:capitalize)].join }
#=> "myVarName"
Where str = 'My-var_name' the result is "MyVarName", since the first element of the splitting result is untouched, while the rest is mapped to be capitalized.
It works only with "dash/underscore delimited words", no spaces, or you need to split by spaces, then map with the presented method.
This method is using string splitting by delimiters, as explained here Split string by multiple delimiters,
chained with Object#then.

How the Anchor \z and \G works in Ruby?

I am using Ruby1.9.3. I am newbie to this platform.
From the doc I just got familiared with two anchor which are \z and \G. Now I little bit played with \z to see how it works, as the definition(End or End of String) made me confused, I can't understand what it meant say - by End. So I tried the below small snippets. But still unable to catch.
CODE
irb(main):011:0> str = "Hit him on the head me 2\n" + "Hit him on the head wit>
=> "Hit him on the head me 2\nHit him on the head with a 24\n"
irb(main):012:0> str =~ /\d\z/
=> nil
irb(main):013:0> str = "Hit him on the head me 24 2\n" + "Hit him on the head >
=> "Hit him on the head me 24 2\nHit him on the head with a 24\n"
irb(main):014:0> str =~ /\d\z/
=> nil
irb(main):018:0> str = "Hit1 him on the head me 24 2\n" + "Hit him on the head>
=> "Hit1 him on the head me 24 2\nHit him on the head with a11 11 24\n"
irb(main):019:0> str =~ /\d\z/
=> nil
irb(main):020:0>
Every time I got nil as the output. So how the calculation is going on for \z ? what does End mean? - I think my concept took anything wrong with the End word in the doc. So anyone could help me out to understand the reason what is happening with the out why so happening?
And also i didn't find any example for the anchor \G . Any example please from you people to make visualize how \G used in real time programming?
EDIT
irb(main):029:0>
irb(main):030:0* ("{123}{45}{6789}").scan(/\G(?!^)\{\d+\}/)
=> []
irb(main):031:0> ('{123}{45}{6789}').scan(/\G(?!^)\{\d+\}/)
=> []
irb(main):032:0>
Thanks
\z matches the end of the input. You are trying to find a match where 4 occurs at the end of the input. Problem is, there is a newline at the end of the input, so you don't find a match. \Z matches either the end of the input or a newline at the end of the input.
So:
/\d\z/
matches the "4" in:
"24"
and:
/\d\Z/
matches the "4" in the above example and the "4" in:
"24\n"
Check out this question for example of using \G:
Examples of regex matcher \G (The end of the previous match) in Java would be nice
UPDATE: Real-World uses for \G
I came up with a more real world example. Say you have a list of words that are separated by arbitrary characters that cannot be well predicted (or there's too many possibilities to list). You'd like to match these words where each word is its own match up until a particular word, after which you don't want to match any more words. For example:
foo,bar.baz:buz'fuzz*hoo-har/haz|fil^bil!bak
You want to match each word until 'har'. You don't want to match 'har' or any of the words that follow. You can do this relatively easily using the following pattern:
/(?<=^|\G\W)\w+\b(?<!har)/
rubular
The first attempt will match the beginning of the input followed by zero non-word character followed by 3 word characters ('foo') followed by a word boundary. Finally, a negative lookbehind assures that the word which has just been matched is not 'har'.
On the second attempt, matching picks back up at the end of the last match. 1 non-word character is matched (',' - though it is not captured due to the lookbehind, which is a zero-width assertion), followed by 3 characters ('bar').
This continues until 'har' is matched, at which point the negative lookbehind is triggered and the match fails. Because all matches are supposed to be "attached" to the last successful match, no additional words will be matched.
The result is:
foo
bar
baz
buz
fuzz
hoo
If you want to reverse it and have all words after 'har' (but, again, not including 'har'), you can use an expression like this:
/(?!^)(?<=har\W|\G\W)\w+\b/
rubular
This will match either a word which is immediately preceeded by 'har' or the end of the last match (except we have to make sure not to match the beginning of the input). The list of matches is:
haz
fil
bil
bak
If you do want to match 'har' and all following words, you could use this:
/\bhar\b|(?!^)(?<=\G\W)\w+\b/
rubular
This produces the following matches:
har
haz
fil
bil
bak
Sounds like you want to know how Regex works? Or do you want to know how Regex works with ruby?
Check these out.
Regexp Class description
The Regex Coach - Great for testing regex matching
Regex cheat sheet
I understand \G to be a boundary match character. So it would tell the next match to start at the end of the last match. Perhaps since you haven't made a match yet you cant have a second.
Here is the best example I can find. Its not in ruby but the concept should be the same.
I take it back this might be more useful

How do I write a regular expression that will match characters in any order?

I'm trying to write a regular expressions that will match a set of characters without regard to order. For example:
str = "act"
str.scan(/Insert expression here/)
would match:
cat
act
tca
atc
tac
cta
but would not match ca, ac or cata.
I read through a lot of similar questions and answers here on StackOverflow, but have not found one that matches my objectives exactly.
To clarify a bit, I'm using ruby and do not want to allow repeat characters.
Here is your solution
^(?:([act])(?!.*\1)){3}$
See it here on Regexr
^ # matches the start of the string
(?: # open a non capturing group
([act]) # The characters that are allowed and a capturing group
(?!.*\1) # That character is matched only if it does not occur once more, Lookahead assertion
){3} # Defines the amount of characters
$
The only special think is the lookahead assertion, to ensure the character is not repeated.
^ and $ are anchors to match the start and the end of the string.
[act]{3} or ^[act]{3}$ will do it in most regular expression dialects. If you can narrow down the system you're using, that will help you get a more specific answer.
Edit: as mentioned by #georgydyer in the comments below, it's unclear from your question whether or not repeated characters are allowed. If not, you can adapt the answer from this question and get:
^(?=[act]{3}$)(?!.*(.).*\1).*$
That is, a positive lookahead to check a match, and then a negative lookahead with a backreference to exclude repeated characters.
Here's how I'd go about it:
regex = /\b(?:#{ Regexp.union(str.split('').permutation.map{ |a| a.join }).source })\b/
# => /(?:act|atc|cat|cta|tac|tca)/
%w[
cat act tca atc tac cta
ca ac cata
].each do |w|
puts '"%s" %s' % [w, w[regex] ? 'matches' : "doesn't match"]
end
That outputs:
"cat" matches
"act" matches
"tca" matches
"atc" matches
"tac" matches
"cta" matches
"ca" doesn't match
"ac" doesn't match
"cata" doesn't match
I use the technique of passing an array into Regexp.union for a lot of things; I works especially well with the keys of a hash, and passing the hash into gsub for rapid search/replace on text templates. This is the example from the gsub documentation:
'hello'.gsub(/[eo]/, 'e' => 3, 'o' => '*') #=> "h3ll*"
Regexp.union creates a regex, and it's important to use source instead of to_s when extracting the actual pattern being generated:
puts regex.to_s
=> (?-mix:\b(?:act|atc|cat|cta|tac|tca)\b)
puts regex.source
=> \b(?:act|atc|cat|cta|tac|tca)\b
Notice how to_s embeds the pattern's flags inside the string. If you don't expect them you can accidentally embed that pattern into another, which won't behave as you expect. Been there, done that and have the dented helmet as proof.
If you really want to have fun, look into the Perl Regexp::Assemble module available on CPAN. Using that, plus List::Permutor, lets us generate more complex patterns. On a simple string like this it won't save much space, but on long strings or large arrays of desired hits it can make a huge difference. Unfortunately, Ruby has nothing like this, but it is possible to write a simple Perl script with the word or array of words, and have it generate the regex and pass it back:
use List::Permutor;
use Regexp::Assemble;
my $regex_assembler = Regexp::Assemble->new;
my $perm = new List::Permutor split('', 'act');
while (my #set = $perm->next) {
$regex_assembler->add(join('', #set));
}
print $regex_assembler->re, "\n";
(?-xism:(?:a(?:ct|tc)|c(?:at|ta)|t(?:ac|ca)))
See "Is there an efficient way to perform hundreds of text substitutions in Ruby?" for more information about using Regexp::Assemble with Ruby.
I will assume several things here:
- You are looking for permutations of given characters
- You are using ruby
str = "act"
permutations = str.split(//).permutation.map{|p| p.join("")}
# and for the actual test
permutations.include?("cat")
It is no regex though.
No doubt - the regex that uses positive/negative lookaheads and backreferences is slick, but if you're only dealing with three characters, I'd err on the side of verbosity by explicitly enumerating the character permutations like #scones suggested.
"act".split('').permutation.map(&:join)
=> ["act", "atc", "cat", "cta", "tac", "tca"]
And if you really need a regex out of it for scanning a larger string, you can always:
Regexp.union "act".split('').permutation.map(&:join)
=> /\b(act|atc|cat|cta|tac|tca)\b/
Obviously, this strategy doesn't scale if your search string grows, but it's much easier to observe the intent of code like this in my opinion.
EDIT: Added word boundaries for false positive on cata based on #theTinMan's feedback.

Remove all non-alphabetical, non-numerical characters from a string?

If I wanted to remove things like:
.!,'"^-# from an array of strings, how would I go about this while retaining all alphabetical and numeric characters.
Allowed alphabetical characters should also include letters with diacritical marks including à or ç.
You should use a regex with the correct character property. In this case, you can invert the Alnum class (Alphabetic and numeric character):
"◊¡ Marc-André !◊".gsub(/\p{^Alnum}/, '') # => "MarcAndré"
For more complex cases, say you wanted also punctuation, you can also build a set of acceptable characters like:
"◊¡ Marc-André !◊".gsub(/[^\p{Alnum}\p{Punct}]/, '') # => "¡MarcAndré!"
For all character properties, you can refer to the doc.
string.gsub(/[^[:alnum:]]/, "")
The following will work for an array:
z = ['asfdå', 'b12398!', 'c98347']
z.each { |s| s.gsub! /[^[:alnum:]]/, '' }
puts z.inspect
I borrowed Jeremy's suggested regex.
You might consider a regular expression.
http://www.regular-expressions.info/ruby.html
I'm assuming that you're using ruby since you tagged that in your post. You could go through the array, put it through a test using a regexp, and if it passes remove/keep it based on the regexp you use.
A regexp you might use might go something like this:
[^.!,^-#]
That will tell you if its not one of the characters inside the brackets. However, I suggest that you look up regular expressions, you might find a better solution once you know their syntax and usage.
If you truly have an array (as you state) and it is an array of strings (I'm guessing), e.g.
foo = [ "hello", "42 cats!", "yöwza" ]
then I can imagine that you either want to update each string in the array with a new value, or that you want a modified array that only contains certain strings.
If the former (you want to 'clean' every string the array) you could do one of the following:
foo.each{ |s| s.gsub! /\p{^Alnum}/, '' } # Change every string in place…
bar = foo.map{ |s| s.gsub /\p{^Alnum}/, '' } # …or make an array of new strings
#=> [ "hello", "42cats", "yöwza" ]
If the latter (you want to select a subset of the strings where each matches your criteria of holding only alphanumerics) you could use one of these:
# Select only those strings that contain ONLY alphanumerics
bar = foo.select{ |s| s =~ /\A\p{Alnum}+\z/ }
#=> [ "hello", "yöwza" ]
# Shorthand method for the same thing
bar = foo.grep /\A\p{Alnum}+\z/
#=> [ "hello", "yöwza" ]
In Ruby, regular expressions of the form /\A………\z/ require the entire string to match, as \A anchors the regular expression to the start of the string and \z anchors to the end.

Remove Duplicate Numbers and Operators from String

I'm trying to take a string which is a simple math expression, remove all spaces, remove all duplicate operators, convert to single digit numbers, and then evaluate.
For example, an string like "2 7+*3*95" should be converted to "2+3*9" and then evaluated as 29.
Here's what I have so far:
expression.slice!(/ /) # Remove whitespace
expression.slice!(/\A([\+\-\*\/]+)/) # Remove operators from the beginning
expression.squeeze!("0123456789") # Single digit numbers (doesn't work)
expression.squeeze!("+-*/") # Removes duplicate operators (doesn't work)
expression.slice!(/([\+\-\*\/]+)\Z/) # Removes operators from the end
puts eval expression
Unfortunately this doesn't make single digit numbers nor remove duplicate operators quite like I expected. Any ideas?
"2 7+*3*95".gsub(/([0-9])[0-9 ]*/, '\1').gsub(/([\+\*\/\-])[ +\*\/\-]+/, '\1')
The first regex handles the single-digit thing and the second handles repeat operators. You could probably condense it into a single regex if you really wanted to.
This works for a quick-and-dirty solution, but you might be better served by a proper parser.
DATA.each { |expr|
expr.gsub!(%r'\s+', '')
expr.gsub!(%r'([*/+-])[*/+-]+', '\1')
expr.gsub!(%r'(\d)\d+', '\1')
expr.sub!(%r'\A[*/+-]+', '')
expr.sub!(%r'[*/+-]+\Z', '')
puts expr + ' = ' + eval(expr).to_s
}
__END__
2 7+*3*95
+-2 888+*3*95+8*-2/+435+-

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