Regular-expression modifiers:
Regular expression literals may include an optional modifier to control various aspects of matching. The modifier is specified after the second slash character, as shown previously and may be represented by one of these characters:
Modifier Description
i Ignore case when matching text.
o Perform #{} interpolations only once, the first time the regexp literal is evaluated.
x Ignores whitespace and allows comments in regular expressions
m Matches multiple lines, recognizing newlines as normal characters
u,e,s,n Interpret the regexp as Unicode (UTF-8), EUC, SJIS, or ASCII.
If none of these modifiers is specified, the regular expression is
assumed to use the source encoding.
Note: that description above has proviso. See sawa's answer for that.