diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/regexp.rdoc | 132 |
1 files changed, 81 insertions, 51 deletions
diff --git a/doc/regexp.rdoc b/doc/regexp.rdoc index 9263e229d8..59f1c45651 100644 --- a/doc/regexp.rdoc +++ b/doc/regexp.rdoc @@ -16,9 +16,12 @@ example: If a string contains the pattern it is said to <i>match</i>. A literal string matches itself. - # 'haystack' does not contain the pattern 'needle', so doesn't match. +Here 'haystack' does not contain the pattern 'needle', so it doesn't match: + /needle/.match('haystack') #=> nil - # 'haystack' does contain the pattern 'hay', so it matches + +Here 'haystack' contains the pattern 'hay', so it matches: + /hay/.match('haystack') #=> #<MatchData "hay"> Specifically, <tt>/st/</tt> requires that the string contains the letter @@ -50,7 +53,7 @@ object. Regexp.last_match is equivalent to <tt>$~</tt>. === Regexp#match method -#match method return a MatchData object : +The #match method returns a MatchData object: /st/.match('haystack') #=> #<MatchData "st"> @@ -108,7 +111,9 @@ operator which performs set intersection on its arguments. The two can be combined as follows: /[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z)) - # This is equivalent to: + +This is equivalent to: + /[abh-w]/ The following metacharacters also behave like character classes: @@ -173,8 +178,9 @@ to occur. Such metacharacters are called <i>quantifiers</i>. * <tt>{</tt><i>n</i><tt>,</tt><i>m</i><tt>}</tt> - At least <i>n</i> and at most <i>m</i> times - # At least one uppercase character ('H'), at least one lowercase - # character ('e'), two 'l' characters, then one 'o' +At least one uppercase character ('H'), at least one lowercase character +('e'), two 'l' characters, then one 'o': + "Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello"> Repetition is <i>greedy</i> by default: as many occurrences as possible @@ -183,9 +189,10 @@ contrast, <i>lazy</i> matching makes the minimal amount of matches necessary for overall success. A greedy metacharacter can be made lazy by following it with <tt>?</tt>. - # Both patterns below match the string. The first uses a greedy - # quantifier so '.+' matches '<a><b>'; the second uses a lazy - # quantifier so '.+?' matches '<a>'. +Both patterns below match the string. The first uses a greedy quantifier so +'.+' matches '<a><b>'; the second uses a lazy quantifier so '.+?' matches +'<a>': + /<.+>/.match("<a><b>") #=> #<MatchData "<a><b>"> /<.+?>/.match("<a><b>") #=> #<MatchData "<a>"> @@ -202,12 +209,15 @@ with <i>n</i>. Within a pattern use the <i>backreference</i> <tt>\n</tt>; outside of the pattern use <tt>MatchData[</tt><i>n</i><tt>]</tt>. - # 'at' is captured by the first group of parentheses, then referred to - # later with \1 +'at' is captured by the first group of parentheses, then referred to later +with <tt>\1</tt>: + /[csh](..) [csh]\1 in/.match("The cat sat in the hat") #=> #<MatchData "cat sat in" 1:"at"> - # Regexp#match returns a MatchData object which makes the captured - # text available with its #[] method. + +Regexp#match returns a MatchData object which makes the captured text +available with its #[] method: + /[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at' Capture groups can be referred to by name when defined with the @@ -239,11 +249,13 @@ also assigned to local variables with corresponding names. Parentheses also <i>group</i> the terms they enclose, allowing them to be quantified as one <i>atomic</i> whole. - # The pattern below matches a vowel followed by 2 word characters: - # 'aen' +The pattern below matches a vowel followed by 2 word characters: + /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen"> - # Whereas the following pattern matches a vowel followed by a word - # character, twice, i.e. <tt>[aeiou]\w[aeiou]\w</tt>: 'enor'. + +Whereas the following pattern matches a vowel followed by a word character, +twice, i.e. <tt>[aeiou]\w[aeiou]\w</tt>: 'enor'. + /([aeiou]\w){2}/.match("Caenorhabditis elegans") #=> #<MatchData "enor" 1:"or"> @@ -252,13 +264,16 @@ capturing. That is, it combines the terms it contains into an atomic whole without creating a backreference. This benefits performance at the slight expense of readability. - # The group of parentheses captures 'n' and the second 'ti'. The - # second group is referred to later with the backreference \2 +The first group of parentheses captures 'n' and the second 'ti'. The second +group is referred to later with the backreference <tt>\2</tt>: + /I(n)ves(ti)ga\2ons/.match("Investigations") #=> #<MatchData "Investigations" 1:"n" 2:"ti"> - # The first group of parentheses is now made non-capturing with '?:', - # so it still matches 'n', but doesn't create the backreference. Thus, - # the backreference \1 now refers to 'ti'. + +The first group of parentheses is now made non-capturing with '?:', so it +still matches 'n', but doesn't create the backreference. Thus, the +backreference <tt>\1</tt> now refers to 'ti'. + /I(?:n)ves(ti)ga\1ons/.match("Investigations") #=> #<MatchData "Investigations" 1:"ti"> @@ -273,14 +288,16 @@ way <i>pat</i> is treated as a non-divisible whole. Atomic grouping is typically used to optimise patterns so as to prevent the regular expression engine from backtracking needlessly. - # The <tt>"</tt> in the pattern below matches the first character of - # the string, then <tt>.*</tt> matches <i>Quote"</i>. This causes the - # overall match to fail, so the text matched by <tt>.*</tt> is - # backtracked by one position, which leaves the final character of the - # string available to match <tt>"</tt> +The <tt>"</tt> in the pattern below matches the first character of the string, +then <tt>.*</tt> matches <i>Quote"</i>. This causes the overall match to fail, +so the text matched by <tt>.*</tt> is backtracked by one position, which +leaves the final character of the string available to match <tt>"</tt> + /".*"/.match('"Quote"') #=> #<MatchData "\"Quote\""> - # If <tt>.*</tt> is grouped atomically, it refuses to backtrack - # <i>Quote"</i>, even though this means that the overall match fails + +If <tt>.*</tt> is grouped atomically, it refuses to backtrack <i>Quote"</i>, +even though this means that the overall match fails + /"(?>.*)"/.match('"Quote"') #=> nil == Subexpression Calls @@ -290,9 +307,10 @@ subexpression named _name_, which can be a group name or number, again. This differs from backreferences in that it re-executes the group rather than simply trying to re-match the same text. - # Matches a <i>(</i> character and assigns it to the <tt>paren</tt> - # group, tries to call that the <tt>paren</tt> sub-expression again - # but fails, then matches a literal <i>)</i>. +This pattern matches a <i>(</i> character and assigns it to the <tt>paren</tt> +group, tries to call that the <tt>paren</tt> sub-expression again but fails, +then matches a literal <i>)</i>: + /\A(?<paren>\(\g<paren>*\))*\z/ =~ '()' @@ -426,15 +444,17 @@ following scripts are supported: <i>Arabic</i>, <i>Armenian</i>, <i>Tamil</i>, <i>Telugu</i>, <i>Thaana</i>, <i>Thai</i>, <i>Tibetan</i>, <i>Tifinagh</i>, <i>Ugaritic</i>, <i>Vai</i>, and <i>Yi</i>. - # Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and - # belongs to the Arabic script. +Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and belongs to the +Arabic script: + /\p{Arabic}/.match("\u06E9") #=> #<MatchData "\u06E9"> All character properties can be inverted by prefixing their name with a caret (<tt>^</tt>). - # Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so - # this match succeeds +Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so this +match succeeds: + /\p{^Ll}/.match("A") #=> #<MatchData "A"> == Anchors @@ -465,22 +485,30 @@ characters, <i>anchoring</i> the match to a specific position. assertion: ensures that the preceding characters do not match <i>pat</i>, but doesn't include those characters in the matched text - # 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: + /real/.match("surrealist") #=> #<MatchData "real"> - # Anchoring the pattern to the beginning of the string forces the - # match to start there. 'real' doesn't occur at the beginning of the - # string, so now the match fails + +Anchoring the pattern to the beginning of the string forces the match to start +there. 'real' doesn't occur at the beginning of the string, so now the match +fails: + /\Areal/.match("surrealist") #=> nil - # The match below fails because although 'Demand' contains 'and', the - pattern does not occur at a word boundary. + +The match below fails because although 'Demand' contains 'and', the pattern +does not occur at a word boundary. + /\band/.match("Demand") - # Whereas in the following example 'and' has been anchored to a - # non-word boundary so instead of matching the first 'and' it matches - # from the fourth letter of 'demand' instead + +Whereas in the following example 'and' has been anchored to a non-word +boundary so instead of matching the first 'and' it matches from the fourth +letter of 'demand' instead: + /\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve"> - # The pattern below uses positive lookahead and positive lookbehind to - # match text appearing in <b></b> tags without including the tags in the - # match + +The pattern below uses positive lookahead and positive lookbehind to match +text appearing in <b></b> tags without including the tags in the match: + /(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>") #=> #<MatchData "bold"> @@ -518,7 +546,8 @@ octothorpe (<tt>#</tt>) character introduces a comment until the end of the line. This allows the components of the pattern to be organised in a potentially more readable fashion. - # A contrived pattern to match a number with optional decimal places +A contrived pattern to match a number with optional decimal places: + float_pat = /\A [[:digit:]]+ # 1 or more digits before the decimal point (\. # Decimal point @@ -634,8 +663,9 @@ backtracking: A similar case is typified by the following example, which takes approximately 60 seconds to execute for me: - # Match a string of 29 <i>a</i>s against a pattern of 29 optional - # <i>a</i>s followed by 29 mandatory <i>a</i>s. +Match a string of 29 <i>a</i>s against a pattern of 29 optional <i>a</i>s +followed by 29 mandatory <i>a</i>s: + Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29 The 29 optional <i>a</i>s match the string, but this prevents the 29 |