Get substring from string using regex in ruby

Question

ex = "g4net:HostName=abc}\n Unhandled Exception: \nSystem.NullReferenceException: Object reference not set to an";
puts ex[/Unhandled Exception:(.*?):/,0]

/Unhandled Exception:(.*?):/ should match \\nSystem.NullReferenceException (as tested in rubular) but it keeps displaying no result.

I'm new to ruby. Please help how can I extract a match for /Unhandled Exception:(.*?):/ from given string

Answer 1

Ruby (and most other languages) use regular expression dialects which do not match newline characters with . by default. In Ruby you can use the m (multiline) modifier:

matchinfo = ex.match(/Unhandled Exception: (.*)/m)
# Allow "." to match newlines ------------------^
matchinfo[1] # => "\nSystem.NullRef..."

You can also use the character class [\\s\\S] instead of . for similar effect, without the need for the multiline modifier:

matchinfo = ex.match(/Unhandled Exception: ([\s\S]*)/)
# Really match *any* character -------------^----^
matchinfo[1] # => "\nSystem.NullRef..."

Answer 2

Running the regex in multiline mode should solve the issue:

(?m)Unhandled Exception:(.*?):

Code:

re = /Unhandled Exception:(.*?):/m
str = 'g4net:HostName=abc}
 Unhandled Exception: 
System.NullReferenceException: Object reference not set to an
'

# Print the match result
str.scan(re) do |match|
    puts match.to_s
end