How do I use a ruby regex to get a substring?

Question

I want to get the numbers out of strings such as:

person_3 person_34 person_356 city_4 city_15
etc...

It seems to me that the following should work:

string[/[0-9]*/]

but this always spits out an empty string.

string[/_.*$/][1..-1] works, but I'm still curious why just asking for the [0-9]* doesn't work... — muirbot
– muirbot, Commented Aug 18, 2010 at 18:33

user229044 · Accepted Answer · 2010-08-18 18:38:15Z

9

[0-9]* successfully matches "0 or more" digits at the beginning of the string, so it returns "". [0-9]+ will match "1 or more" digits, and works as you expect:

irb(main):001:0> x = "test 92"
=> "test 92"
irb(main):003:0> x[/\d*/]
=> ""
irb(main):005:0> x.index(/\d*/)
=> 0
irb(main):004:0> x[/\d+/]
=> "92"

edited Aug 18, 2010 at 18:38

answered Aug 18, 2010 at 18:32

user229044♦

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2 Comments

Mladen Jablanović Over a year ago

It's not because of non-greedy matching (which it isn't), but because * matches zero or more chars. It finds zero digits in the beginning of the string, and returns them. You can test this by trying the same regex on string "34_person", /[0-9]*/ will return both digits.

muirbot Over a year ago

Its not being greedy, its being lazy! Thanks to both.

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