116

Is is possible to create/use a regular expression pattern in ruby that is based on the value of a variable name?

For instance, we all know we can do the following with Ruby strings:

str = "my string"
str2 = "This is #{str}" # => "This is my string"

I'd like to do the same thing with regular expressions:

var = "Value"
str = "a test Value"
str.gsub( /#{var}/, 'foo' ) # => "a test foo"

Obviously that doesn't work as listed, I only put it there as an example to show what I'd like to do. I need to regexp match based on the value of a variable's content.

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5 Answers 5

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207

The code you think doesn't work, does:

var = "Value"
str = "a test Value"
p str.gsub( /#{var}/, 'foo' )   # => "a test foo"

Things get more interesting if var can contain regular expression meta-characters. If it does and you want those matacharacters to do what they usually do in a regular expression, then the same gsub will work:

var = "Value|a|test"
str = "a test Value"
str.gsub( /#{var}/, 'foo' ) # => "foo foo foo"

However, if your search string contains metacharacters and you do not want them interpreted as metacharacters, then use Regexp.escape like this:

var = "*This*"
str = "*This* is a string"
p str.gsub( /#{Regexp.escape(var)}/, 'foo' )
# => "foo is a string"

Or just give gsub a string instead of a regular expression. In MRI >= 1.8.7, gsub will treat a string replacement argument as a plain string, not a regular expression:

var = "*This*"
str = "*This* is a string"
p str.gsub(var, 'foo' ) # => "foo is a string"

(It used to be that a string replacement argument to gsub was automatically converted to a regular expression. I know it was that way in 1.6. I don't recall which version introduced the change).

As noted in other answers, you can use Regexp.new as an alternative to interpolation:

var = "*This*"
str = "*This* is a string"
p str.gsub(Regexp.new(Regexp.escape(var)), 'foo' )
# => "foo is a string"
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3 Comments

The hint on Regexp.escape was just what I didn't know I needed. Thanks!
regxPlayerVariable = '(.*?)=.*?document\.getElementById(#{pluginPlayeVariable})' in this i applied all above but these are not working.
@SSP It might be good to explain the trouble in a separate question.
13

It works, but you need to use gsub! or assign the return to another variable

var = "Value"
str = "a test Value"
str.gsub!( /#{var}/, 'foo' )  # Or this: new_str = str.gsub( /#{var}/, 'foo' )
puts str
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11

Yes

str.gsub Regexp.new(var), 'foo'
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7

You can use regular expressions through variables in ruby:

var = /Value/
str = "a test Value"
str.gsub( /#{var}/, 'foo' )
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2 Comments

I'm not sure why this only has downvotes. I tried it and it works, even though you don't need the regex slashes in both the var variable and gsub's first parameter.
I think the point of the slashes in gsub was to show that a regex can contain a variable which is itself a regex. The variable doesn't have to be string type (and in fact, regex is better for reasons outlined in Wayne's answer).
3 
str.gsub( Regexp.new("#{var}"), 'foo' )
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2 Comments

I think it's Regexp
Yes, it is Regexp ruby-doc.org/core-2.7.0/Regexp.html

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