Ruby regular expression using variable name

Question

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.

Wayne Conrad · Accepted Answer · 2013-08-23 21:04:54Z

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"

edited Aug 23, 2013 at 21:04

answered Feb 15, 2010 at 20:07

Wayne Conrad

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3 Comments

Jeff Paquette Over a year ago

The hint on Regexp.escape was just what I didn't know I needed. Thanks!

SSP Over a year ago

regxPlayerVariable = '(.*?)=.*?document\.getElementById(#{pluginPlayeVariable})' in this i applied all above but these are not working.

Wayne Conrad Over a year ago

@SSP It might be good to explain the trouble in a separate question.

FMc · Accepted Answer · 2010-02-15 19:38:26Z

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

answered Feb 15, 2010 at 19:38

FMc

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Comments

DigitalRoss · Accepted Answer · 2010-02-15 19:39:49Z

11

Yes

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

answered Feb 15, 2010 at 19:39

DigitalRoss

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Comments

Adam Ocsvari · Accepted Answer · 2015-07-31 16:33:27Z

7

You can use regular expressions through variables in ruby:

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

edited Jul 31, 2015 at 16:33

Adam Ocsvari

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answered Feb 15, 2010 at 19:39

Dev Sahoo

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2 Comments

Tyler Collier Over a year ago

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.

mahemoff Over a year ago

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).

DNNX · Accepted Answer · 2020-10-30 15:45:34Z

3

str.gsub( Regexp.new("#{var}"), 'foo' )

edited Oct 30, 2020 at 15:45

answered Feb 15, 2010 at 19:39

DNNX

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2 Comments

rj487 Over a year ago

I think it's Regexp

Edgar Ortega Over a year ago

Yes, it is Regexp ruby-doc.org/core-2.7.0/Regexp.html

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