Ruby gsub / regex with several arguments [duplicate] - ruby

Question

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Match a string against multiple patterns
(2 answers)
Closed 8 years ago.
I'm new to ruby and I'm trying to solve a problem.
I'm parsing through several text field where I want to remove the header which has different values. It works fine when the header always is the same:
variable = variable.gsub(/(^Header_1:$)/, '')
But when I put in several arguments it doesn't work:
variable = variable.gsub(/(^Header_1$)/ || /(^Header_2$)/ || /(^Header_3$)/ || /(^Header_4$)/ || /^:$/, '')

Answer 1

You can use Regexp.union:
regex = Regexp.union(
/^Header_1/,
/^Header_2/,
/^Header_3/,
/^Header_4/,
/^:$/
)
variable.gsub(regex, '')
Please note that ^something$ will not work on strings containing something more than something :)
Cause ^ is for matching beginning of string and $ is for end of string.
So i intentionally removed $.
Also you do not need brackets when you only need to remove the matched string.
You can also use it like this:
headers = %w[Header_1 Header_2 Header_3]
regex = Regexp.union(*headers.map{|s| /^#{s}/}, /^\:$/, /etc/)
variable.gsub(regex, '')
And of course you can remove headers without explicitly define them.
Most likely there are a white space after headers?
If so, you can do it as simple as:
variable = "Header_1 something else"
puts variable.gsub(/(^Header[^\s]*)?(.*)/, '\2')
#=> something else
variable = "Header_BLAH something else"
puts variable.gsub(/(^Header[^\s]*)?(.*)/, '\2')
#=> something else

Answer 2

Just use a proper regexp:
variable.gsub(/^(Header_1|Header_2|Header_3|Header_4|:)$/, '')

Answer 3

If the header is always the same format of Header_n, where n is some integer value, then you can simplify your regex greatly:
/Header_\d+/
will find every one of these:
%w[Header_1 Header_2 Header_3].grep(/Header_\d+/)
[
[0] "Header_1",
[1] "Header_2",
[2] "Header_3"
]
Tweaking it to handle finding words, not substrings:
/^Header_\d+$/
or:
/\bHeader_\d+\b/
As mentioned, using Regexp.union is a good start, but, used blindly, can result in very slow or inefficient patterns, so think ahead and help out the engine by giving it useful sub-patterns to work with:
values = %w[foo bar]
/Header_(?:\d+|#{ values.join('|') })/
=> /Header_(?:\d+|foo|bar)/
Unfortunately, Ruby doesn't have the equivalent to Perl's Regexp::Assemble module, which can build highly optimized patterns from big lists of words. Search here on Stack Overflow for examples of what it can do. For instance:
use Regexp::Assemble;
my #values = ('Header_1', 'Header_2', 'foo', 'bar', 'Header_3');
my $ra = Regexp::Assemble->new;
foreach (#values) {
$ra->add($_);
}
print $ra->re, "\n";
=> (?-xism:(?:Header_[123]|bar|foo))