Ruby gsub / regex modifiers?

Question

Where can I find the documentation on the modifiers for gsub? \a \b \c \1 \2 \3 %a %b %c $1 $2 %3 etc.?

Specifically, I'm looking at this code... something.gsub(/%u/, unit) what's the %u?

Answer 1

Zenspider's Quickref contains a section explaining which escape sequences can be used in regexen and one listing the pseudo variables that get set by a regexp match. In the second argument to gsub you simply write the name of the variable with a backslash instead of a $ and it will be replaced with the value of that variable after applying the regexp. If you use a double quoted string, you need to use two backslashes.

When using the block-form of gsub you can simply use the variables directly. If you return a string containing e.g. \1 from the block, that will not be replaced with $1. That only happens when using the two-argument form.

Answer 2

Hi Blaine,

First off, %u is nothing special in ruby regex:

mixonic@pandora ~ $ irb
irb(main):001:0> '%u'.gsub(/%u/,'heyhey')
=> "heyhey"

The definitive documentation for Ruby 1.8 regex is in the Ruby Doc Bundle:

• http://ruby-doc.org/docs/ruby-doc-bundle/Manual/man-1.4/syntax.html#regexp

Strings delimited by slashes are regular expressions. The characters right after latter slash denotes the option to the regular expression. Option i means that regular expression is case insensitive. Option i means that regular expression does expression substitution only once at the first time it evaluated. Option x means extended regular expression, which means whitespaces and commens are allowd in the expression. Option p denotes POSIX mode, in which newlines are treated as normal character (matches with dots).

The %r/STRING/ is the another form of the regular expression.

^
    beginning of a line or string 
$
    end of a line or string 
.
    any character except newline 
\w
    word character[0-9A-Za-z_] 
\W
    non-word character 
\s
    whitespace character[ \t\n\r\f] 
\S
    non-whitespace character 
\d
    digit, same as[0-9] 
\D
    non-digit 
\A
    beginning of a string 
\Z
    end of a string, or before newline at the end 
\z
    end of a string 
\b
    word boundary(outside[]only) 
\B
    non-word boundary 
\b
    backspace(0x08)(inside[]only) 
[ ]
    any single character of set 
*
    0 or more previous regular expression 
*?
    0 or more previous regular expression(non greedy) 
+
    1 or more previous regular expression 
+?
    1 or more previous regular expression(non greedy) 
{m,n}
    at least m but most n previous regular expression 
{m,n}?
    at least m but most n previous regular expression(non greedy) 
?
    0 or 1 previous regular expression 
|
    alternation 
( )
    grouping regular expressions 
(?# )
    comment 
(?: )
    grouping without backreferences 
(?= )
    zero-width positive look-ahead assertion 
(?! )
    zero-width negative look-ahead assertion 
(?ix-ix)
    turns on (or off) `i' and `x' options within regular expression.

These modifiers are localized inside an enclosing group (if any). (?ix-ix: ) turns on (or off) i' and x' options within this non-capturing group.

Backslash notation and expression substitution available in regular expressions.

Good luck!

Answer 3

For Ruby 1.9's Oniguruma there is a good documentation of the regular expression here.

Answer 4

If you use block in sub/gsub you can access to the groups like that :

>> rx = /(ab(cd)ef)/
>> s = "-abcdef-abcdef"
>> s.gsub(rx) { $2 }
=> "cdgh-cdghi"