This irb session example helps demonstrate repetition in regular expressions using * ? + {}

Definitions:

? - the preceding token is optional
+ - find the token 1 or more times
* - find the token 0 or more times
{n} - repeat the token n no of times

irb(main):058:0> "this song is my favourite"[/favou?rite/]
=> "favourite"
irb(main):059:0> "this song is my favorite"[/favou?rite/]
=> "favorite"

irb(main):100:0> "this is my favourite\s"[/this\s((song|video)\s)?is/]
=> "this is"
irb(main):101:0> "this song is my favourite\s"[/this\s((song|video)\s)?is/]
=> "this song is"
irb(main):102:0> "this flower is my favourite\s"[/this\s((song|video)\s)?is/]
=> nil
irb(main):103:0> "this video is my favourite\s"[/this\s((song|video)\s)?is/]
=> "this video is"

irb(main):105:0> "anything you say"[/.*/]
=> "anything you say"
irb(main):119:0> "will be repeated"[/.*/]
=> "will be repeated"
irb(main):116:0> "good times</p>"[/.*[^<\/p>]/]
=> "good times"

irb(main):220:0> "test 123 test 123"[/\d+/]
=> "123"
irb(main):224:0> "test 123 test 123"[/\d{2}/]
=> "12"
irb(main):226:0> "test 123 test 123"[/\w+/]
=> "test"

irb(main):008:0> "try this test that"[/try.\b\w+/]
=> "try this"
irb(main):009:0> "try this test that"[/test.\b\w+/]
=> "test that"

irb(main):233:0> "test 123 test 123"[/\w+\s\d+/]
=> "test 123"
irb(main):248:0> "test 123 test 123"[/\w+\s\d+.*/]
=> "test 123 test 123"
irb(main):249:0> "test this test 123"[/\w+\s\d+.*/]
=> "test 123"
irb(main):250:0> "test this test that"[/\w+\s\d+.*/]
=> nil
irb(main):279:0> "try this test that"[/try\s\w[^\s]+/]
=> "try this"
irb(main):280:0> "try this test that"[/test\s\w[^\s]+/]
=> "test that"

irb(main):310:0> "try this test that"[/test\s\w\B+/]
=> "test tha"
irb(main):308:0> "try this test that"[/try\s\w\B+./]
=> "try this"
irb(main):309:0> "try this test that"[/test\s\w\B+./]
=> "test that"

irb(main):322:0> "try this test that"[/try\s\w+\b/]
=> "try this"
irb(main):323:0> "try this test that"[/test\s\w+\b/]
=> "test that"

irb(main):008:0> "try this test that"[/try.\b\w+/]
=> "try this"
irb(main):009:0> "try this test that"[/test.\b\w+/]
=> "test that"

irb(main):010:0> "try this test that"[/test.\w+/]
=> "test that"
irb(main):011:0> "try this test that"[/try.\w+/]
=> "try this"

irb(main):012:0> "try this test that"[/try\s\w+/]
=> "try this"
irb(main):013:0> "try this test that"[/test\s\w+/]
=> "test that"

irb(main):015:0> "try this test that"[/try\s\w+\s\w+/]
=> "try this test"

irb(main):041:0> "try this test that"[/\w+\s\w+$/]
=> "test that"
irb(main):043:0> "try this test that"[/^\w+\s\w+/]
=> "try this"

Note: Similar to CSS the more specific an expression or selector is the less ambiguous it is.