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Update Capturing and Anchors sections of regexp documention

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Document that only first 9 numbered capture groups can use the \n
backreference syntax.  Document \0 backreference.  Document \K anchor.

Fixes [Bug #14500]
This commit is contained in:
Jeremy Evans 2021-07-27 12:30:43 -07:00
parent 35e467080c
commit 4fc9ddd7b6
1 changed files with 31 additions and 5 deletions
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doc/regexp.rdoc
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@ -222,13 +222,13 @@ jeopardises the overall match.
== Capturing == Capturing
Parentheses can be used for <i>capturing</i>. The text enclosed by the Parentheses can be used for <i>capturing</i>. The text enclosed by the
<i>n</i><sup>th</sup> group of parentheses can be subsequently referred to <i>n</i>th group of parentheses can be subsequently referred to
with <i>n</i>. Within a pattern use the <i>backreference</i> with <i>n</i>. Within a pattern use the <i>backreference</i>
<tt>\n</tt>; outside of the pattern use <tt>\n</tt> (e.g. <tt>\1</tt>); outside of the pattern use
<tt>MatchData[</tt><i>n</i><tt>]</tt>. <tt>MatchData[n]</tt> (e.g. <tt>MatchData[1]</tt>).
'at' is captured by the first group of parentheses, then referred to later In this example, <tt>'at'</tt> is captured by the first group of
with <tt>\1</tt>: parentheses, then referred to later with <tt>\1</tt>:
/[csh](..) [csh]\1 in/.match("The cat sat in the hat") /[csh](..) [csh]\1 in/.match("The cat sat in the hat")
#=> #<MatchData "cat sat in" 1:"at"> #=> #<MatchData "cat sat in" 1:"at">
@ -238,6 +238,21 @@ available with its #[] method:
/[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at' /[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at'
While Ruby supports an arbitrary number of numbered captured groups,
only groups 1-9 are supported using the <tt>\n</tt> backreference
syntax.
Ruby also supports <tt>\0</tt> as a special backreference, which
references the entire matched string. This is also available at
<tt>MatchData[0]</tt>. Note that the <tt>\0</tt> backreference cannot
be used inside the regexp, as backreferences can only be used after the
end of the capture group, and the <tt>\0</tt> backreference uses the
implicit capture group of the entire match. However, you can use
this backreference when doing substitution:
"The cat sat in the hat".gsub(/[csh]at/, '\0s')
# => "The cats sats in the hats"
=== Named captures === Named captures
Capture groups can be referred to by name when defined with the Capture groups can be referred to by name when defined with the
@ -524,6 +539,17 @@ characters, <i>anchoring</i> the match to a specific position.
* <tt>(?<!</tt><i>pat</i><tt>)</tt> - <i>Negative lookbehind</i> * <tt>(?<!</tt><i>pat</i><tt>)</tt> - <i>Negative lookbehind</i>
assertion: ensures that the preceding characters do not match assertion: ensures that the preceding characters do not match
<i>pat</i>, but doesn't include those characters in the matched text <i>pat</i>, but doesn't include those characters in the matched text
* <tt>\K</tt> - Uses an positive lookbehind of the content preceding
<tt>\K</tt> in the regexp. For example, the following two regexps are
almost equivalent:
/ab\Kc/
/(?<=ab)c/
As are the following two regexps:
/(a)\K(b)\Kc/
/(?<=(?<=(a))(b))c/
If a pattern isn't anchored it can begin at any point in the string: If a pattern isn't anchored it can begin at any point in the string: