Ruby, True/false regex - ruby

So I've got an issue where my regex looks like this: /true|false/.
When I check the word falsee I get a true from this regex, is there a way to just limit it to the exact true or false words?


Use this regex:
/^(true|false)$/
It will match the beginning and end of the test string with ^ and $, respectively, so nothing else can be in the string (exact match).
See live example at Regex101.
UPDATE (see #w0lf's comment): The parentheses are to isolate the true|false clause so that they are not grouped incorrectly. (This also puts the true or false match in the first capturing group, but since it seems that you are only matching and not capturing an output, this should not make a difference).
Alternatively, if you simply want to match two values, there are easier ways in Ruby. #SimoneCarletti suggests one. You can also use the basic == or eql? operators. Try running the following script to see that these all work:
values = ["true", "false", "almosttrue", "falsealmost"]
values.each do | value |
puts value
# these three are all equivalent
puts "match with if" if value == "true" || value == "false"
puts "match with equals?" if (value.eql? "true") || (value.eql? "false")
puts "match with regex" if /^(true|false)$/.match value
puts
end


You need to use the ^ and $ anchors:
/^(true|false)$/
Edit: As Cary pointed out in the comments, the pattern above will also match multiline strings that happen to contain a line with true or false. To avoid this, use the \A and \z delimiters that match the beginning and end of string respectively:
/\A(true|false)\z/


Try out
/^(true|false)$/
where ^ is the start of a line and $ the end.


You can use
/^(true|false)$/
or even better
/\A(true|false)\z/
that will match the beginning and end of the string (instead of line). If you only need to match for whose words, it may be more efficient to use a simple array and include?:
%w( true false ).include?(value)

Related

How to remove strings that end with a particular character in 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

regex scan only returning first value

I have two strings that should both return matches according to the regex, but only str1 returns the expected match. str1 is an exact match for the regex (created by Avinash Raj) below. str2 contains str1 and more data. I expected str2 to return str1 and more values that matched, but it returns nothing Can someone explain why?
str1="3,15,14,31,40,5,5,4,5,3,4,4,5,2,2,2,1,2,1,1,3,3,3,2,4,3,false,false,false,false,false,true,false,true,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,3,3,3,2,3"
str2="3,15,14,31,40,5,5,4,5,3,4,4,5,2,2,2,1,2,1,1,3,3,3,2,4,3,false,false,false,false,false,true,false,true,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,3,3,3,2,3,3,15,14,35,27,4,5,3,5,3,2,4,4,2,1,1,2,2,2,1,3,3,3,2,5,9,true,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,true,true,false,false,false,false,2,2,3,2,3,3,15,16,34,53,4,4,4,3,1,3,4,3,1,1,1,1,1,1,1,2,3,2,3,5,1,true,false,false,false,false,false,true,false,false,false,false,false,false,false,true,true,false,false,false,false,false,false,false,false,false,false,false,3,2,3,2,3,3,15,18,37,29,4,4,4,3,2,3,3,4,1,1,1,1,1,1,1,1,3,1,2,4,1,true,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,3,2,3,2,3,3,15,20,34,37,4,4,4,3,1,3,3,4,1,1,1,1,1,1,1,1,1,1,2,4,1,false,false,false,true,false,false,false,false,false,false,false,false,false,true,false,true,false,false,false,false,false,false,false,false,true,false,false,3,1,3,1,3,3,16,10,18,30,4,3,3,3,1,3,3,3,1,1,1,1,1,1,1,1,2,1,4,4,3,false,false,false,false,false,true,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,3,2,3,2,3,3,16,12,39,5,5,5,4,5,3,5,5,5,1,1,1,1,1,1,1,2,1,1,1,5,10,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,3,2,3,2,3,3,16,14,18,27,4,4,4,4,2,3,3,4,1,1,1,1,1,1,1,1,1,1,2,5,1,true,false,false,false,false,false,false,false,false,false,false,false,false,true,false,true,false,false,false,false,true,false,false,false,false,false,false,3,2,3,2,3,3,16,16,18,32,5,5,5,5,4,5,5,5,1,1,1,1,1,1,1,2,1,1,1,5,3,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,true,false,3,2,3,2,3,3,16,18,20,7,5,5,5,5,3,3,3,4,1,1,1,1,1,1,1,1,1,1,2,5,1,false,false,false,true,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,true,false,false,false,false,false,false,3,2,3,2,3,3,16,20,18,59,4,4,4,3,1,1,1,2,1,1,1,1,1,1,1,1,2,2,4,5,9,false,false,false,true,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,3,2,3,2,3,3,17,10,16,9,3,3,3,3,1,2,3,3,1,1,1,1,1,1,1,1,2,1,3,5,1,true,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,3,2,3,2,3,3,17,12,16,17,4,3,4,2,1,4,3,2,1,1,1,1,1,1,1,1,1,1,4,5,3,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,3,2,3,2,3,3,17,14,16,21,4,4,4,4,1,3,4,4,1,1,1,1,1,1,1,1,1,1,2,5,1,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,true,false,false,3,2,3,2,3,3,17,16,16,20,5,5,4,5,3,4,4,5,1,1,1,1,1,1,1,1,1,1,1,5,8,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,true,false,false,false,3,2,3,2,3,3,17,18,16,31,4,4,4,4,1,4,3,3,1,1,1,1,1,1,1,1,1,1,3,5,1,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,true,false,true,false,false,false,false,3,2,3,2,3,3,17,20,18,8,5,5,4,5,4,4,4,5,1,1,1,1,1,1,1,2,1,1,1,5,1,false,false,false,true,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,true,false,false,false,3,2,3,2,3,3,18,10,31,33,3,2,3,2,2,2,2,3,1,1,1,1,1,1,1,1,1,1,1,5,7,true,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,3,2,3,2,3,3,18,12,36,11,4,4,4,5,3,4,3,3,1,1,2,1,2,1,2,2,1,1,1,5,1,false,false,false,true,false,false,true,false,false,false,false,false,false,true,false,true,false,false,false,false,false,false,false,false,true,false,false,3,2,3,2,3,3,18,14,49,6,3,3,2,2,1,2,2,2,2,1,1,1,2,1,2,3,3,4,4,5,9,true,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,3,2,3,2,3,3,18,16,32,53,3,4,4,3,3,3,3,3,1,1,1,1,1,1,2,2,1,1,3,5,7,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,true,false,false,false,false,3,2,3,2,3,3,18,18,37,59,5,4,4,4,4,4,4,4,1,1,1,1,1,1,1,2,1,1,2,5,7,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,true,false,false,false,false,3,2,3,2,3,3,19,10,5,25,4,4,4,2,2,4,3,3,1,1,1,1,1,1,1,1,2,2,2,5,1,true,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,2,1,3,2,3,3,19,13,0,5,5,5,4,5,3,3,5,5,1,1,1,1,1,1,1,1,1,1,3,5,7,false,false,true,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,false,false,true,false,false,false,false,false,false,3,2,3,2,3,3,19,14,5,23,4,4,4,4,3,4,3,3,1,1,1,1,1,1,1,1,1,2,2,5,9,false,false,true,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,false,3,2,3,2,3,3,19,16,7,19,5,4,4,4,3,4,3,3,1,1,1,1,1,1,1,2,2,2,3,5,9,false,false,true,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,3,2,3,2,3,3,19,18,6,30,4,4,4,4,3,4,4,4,1,1,1,1,1,1,1,1,1,1,1,5,8,false,false,true,true,false,false,false,false,false,false,false,false,false,false,false,false,false,false,true,false,true,false,false,false,true,false,false,3,2,3,2,3,3,19,20,8,25,4,4,5,4,3,4,3,4,1,1,1,1,1,1,1,1,1,1,3,5,1,false,false,true,false,false,false,true,false,false,false,false,false,false,true,false,false,false,false,false,false,false,false,false,false,true,false,false,3,2,3,2,3,3,19,21,18,2,4,4,4,3,3,4,3,4,1,1,1,1,1,1,1,1,1,1,1,5,1,false,false,true,false,false,false,false,false,false,false,false,false,false,true,false,false,false,false,true,false,true,false,false,false,true,false,false,3,2,3,2,3,"
str1.scan(/^,?(?:[1-5]\d|[1-9])(?:,(?:[1-5]\d|[1-9])){4}(?:,[1-5]){21}(?:,(?:true|false)){27}(?:,[1-5]){5}$/).each{|x|
puts x
puts "---1---"
}
str2.scan(/^,?(?:[1-5]\d|[1-9])(?:,(?:[1-5]\d|[1-9])){4}(?:,[1-5]){21}(?:,(?:true|false)){27}(?:,[1-5]){5}$/).each{|x|
puts x
puts "---2---"
}

Kind of by definition, you can't have more than one pattern match in a string when your pattern specifically says "start of string, then [stuff], then end of string". Look at regexp anchors ^ and $.
A simpler example might make it clearer: ^a$ "start of string, then letter a, then end of string" will match in "a" once, but will match in "aaa" zero times, even though there are three letters a.

$ assert position at end of a line
Now you are not matching upto the end of line.
^,?(?:[1-5]\d|[1-9])(?:,(?:[1-5]\d|[1-9])){4}(?:,[1-5]){21}(?:,(?:true|false)){27}(?:,[1-5]){5}
Just remove the $ from the end.See demo.
https://regex101.com/r/sJ9gM7/22

Because you're regular starts with the ^ metacharacter and ends with the $ metacharacter, it expects the full string to match.

How to ignore uppercase using start_with?

Is there a better way to ignore uppercase than this?
"Hello".start_with?("hell","Hell") #=> true
I want to check if a string element in an array starts with another string ignoring uppercase, like LIKE % in MySQL.

I would do something like this:
'Hello'.upcase.start_with?('HELL')
Another approach to the same problem. That's equivalent to do something like UPPER(column) like 'SOMETHING%' in SQL.

You could use a regular expression with String#=~:
"Hello" =~ /^hell/i #=> 0
"hELLO" =~ /^hell/i #=> 0
"world" =~ /^hell/i #=> nil
Since 0 is truthy and nil is falsy this can be used in an if clause:
if str =~ /^hell/i
# starts with hell
end

I think the best is to use Ruby's regex matching with ignore case flag:
'Hello'.match /^hell/i
The '^' designates the start of the string. Without it would match 'hell' anywhere in the string. And the last 'i' is just a regex flag to indicate matching with ignore case set.
You can find more info on Ruby Regex API here:
http://www.regular-expressions.info/ruby.html

Understanding how pattern matching works in Ruby 2

I don't know how pattern matching works in Ruby 2.
I have the following value, targetfilename = /mnt/usb/mpeg4Encoded.mpeg4
My pattern matching if-else is thus:
if (targetfilename.match(/^\//))
puts "amit"
else
puts "ramit"
The output is ramit.
I don't understand how this pattern matching works though.

if targetfilename.match(/^V/)
puts "amit"
else
puts "ramit"
end
# result:
# "amit"
Why is this? This is because targetfilename.match(/^V/) outputs a Matchdata object (click on the link for a full description of this object). This is an object that contains all of the information that is in the "matching". If there is no match, no MatchData object is returned, because there's nothing to return. Instead, you get nil.
When you use if, if it tries to compare a nil, it treats it the same way as false.
Basically, any "actual" value (besides false) is treated the same way as true. Basically, it's asking
if (there's anything here)
do_this
else
do_something_else
end
Again, let me reiterate:
If the thing after if is either false or nil, the if statement resolves to the "else".
If it's anything else, it resolves as if it had gotten a "true" statement.
Regular Expressions
/^V/ is what is called a "Regular Expression"; the // is a Regexp literal the same way that the "" is a String literal, and Regexps are represented by the Regexp class the same way that strings are represented by the String class.
The actual "regular expression" is what's between the slashes -- ^V. This is saying:
^: the start of a string
V: a capital letter V
So, /^V/ will match any cases of the capital letter "V" at the beginning of a string.
What else can you put in a regular expression? What are the special characters? Try this regexp cheat sheet
Also, some great tools:
Rubular -- enter in your regular expression, and then a same text, and see what matches.
Strfriend -- enter in a regular expression and see it "visually" represented.

Regular expression for not matching two underscores

I don't know whether it's really easy and I'm out of my mind....
In Ruby's regular expressions, how to match strings which do not contain two consecutive underscores, i.e., "__".
Ex:
Matches: "abcd", "ab_cd", "a_b_cd", "%*##_#+"
Does not match: "ab__cd", "a_b__cd"
-thanks
EDIT: I can't use reverse logic, i.e., checking for "__" strings and excluding them, since need to use with Ruby on Rails "validates_format_of()" which expects a regular expression with which it will match.

You could use negative lookahead:
^((?!__).)*$
The beginning-of-string ^ and end of string $ are important, they force a check of "not followed by double underscore" on every position.

/^([^_]*(_[^_])?)*_?$/
Tests:
regex=/^([^_]*(_[^_])?)*_?$/
# Matches
puts "abcd" =~ regex
puts "ab_cd" =~ regex
puts "a_b_cd" =~ regex
puts "%*##_#+" =~ regex
puts "_" =~ regex
puts "_a_" =~ regex
# Non-matches
puts "__" =~ regex
puts "ab__cd" =~ regex
puts "a_b__cd" =~ regex
But regex is overkill for this task. A simple string test is much easier:
puts ('a_b'['__'])

Would altering your logic still be valid?
You could check if the string contains two underscores with the regular expression [_]{2} and then just ignore it?

Negative lookahead
\b(?!\w*__\w*)\w+\b
Search for two consecutive underscores in the next word from the beginning of the word, and match that word if it is not found.
Edit: To accommodate anything other than whitespaces in the match:
(?!\S*__\S*)\S+
If you wish to accommodate a subset of symbols, you can write something like the following, but then it will match _cd from a_b__cd among other things.
(?![a-zA-Z0-9_%*##+]*__[a-zA-Z0-9_%*##+]*)[a-zA-Z0-9_%*##+]+

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