Dynamic Regex in Ruby

Question

I am trying to find a way to let me dynamically create a regexp object from a string (taken from the database) and then use that to filter another string. This example is to extract data from a git commit message, but in theory any valid regexp could be present in the database as a string.

What happens

>> string = "[ALERT] Project: Revision ...123456 committed by Me <[email protected]>\n on 2009-   07-28 21:21:47\n\n    Fixed typo\n"
>> r = Regexp.new("[A-Za-z]+: Revision ...[\w]+ committed by [A-Za-z\s]+")
>> string[r]
=> nil

What I want to happen

>> string = "[ALERT] Project: Revision ...123456 committed by Me <[email protected]>\n on 2009-   07-28 21:21:47\n\n    Fixed typo\n"
>> string[/[A-Za-z]+: Revision ...[\w]+ committed by [A-Za-z\s]+/]
=> "Project: Revision 123456 committed by Me"

Gareth · Accepted Answer · 2009-08-01 08:07:27Z

12

You're only missing one thing:

>> Regexp.new "\w"
=> /w/
>> Regexp.new "\\w"
=> /\w/

Backslashes are escape characters in strings. If you want a literal backslash you have to double it.

>> string = "[ALERT] Project: Revision ...123456 committed by Me <[email protected]>\n on 2009-   07-28 21:21:47\n\n    Fixed typo\n"
=> "[ALERT] Project: Revision ...123456 committed by Me <[email protected]>\n on 2009-   07-28 21:21:47\n\n    Fixed typo\n"
>> r = Regexp.new("[A-Za-z]+: Revision ...[\\w]+ committed by [A-Za-z\\s]+")
=> /[A-Za-z]+: Revision ...[\w]+ committed by [A-Za-z\s]+/
>> string[r]
=> "Project: Revision ...123456 committed by Me "

Typically, if you'd pasted the output from your "broken" lines, rather than just the input, you'd probably have spotted that the w and s weren't escaped properly

answered Aug 1, 2009 at 8:07

Gareth

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1 Comment

davidsmalley Over a year ago

Perfect, thanks - I knew I had to be doing something subtly wrong.

Swanand · Accepted Answer · 2009-08-01 08:08:14Z

0

Option 1:

# Escape the slashes:
r = Regexp.new("[A-Za-z]+: Revision ...[\\w]+ committed by [A-Za-z\\s]+")

Disadvantage: manually escape all known escape characters

Option 2:

# Use slashes in constructor
r = Regexp.new(/[A-Za-z]+: Revision ...[\w]+ committed by [A-Za-z\s]+/)

Disadvantage: None

answered Aug 1, 2009 at 8:08

Swanand

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1 Comment

davidsmalley Over a year ago

For option 2 - the argument to the constructor is always string because the regex is being pulled from the database so that won't work in this scenario.

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