|
| 1 | +--- |
| 2 | +title: Regular Extremism |
| 3 | +date: 2015-05-11 |
| 4 | +tags: strings, rexex |
| 5 | +--- |
| 6 | + |
| 7 | +You are here for a collection of 10 advanced features of regular expressions in Ruby! |
| 8 | + |
| 9 | +ARTICLE |
| 10 | + |
| 11 | +## Regex Conditionals |
| 12 | + |
| 13 | +Regular expressions can have embedded conditionals (*if-then-else*) with `(?ref)then|else`. "ref" stands for a group reference (number or name of a capture group): |
| 14 | + |
| 15 | + # will match everything if string contains "ä", or only match first two chars |
| 16 | + regex = /(?=(.*ä))?(?(1).*|..)/ |
| 17 | + |
| 18 | + "Ruby"[regex] #=> "Ru" |
| 19 | + "Idiosyncrätic"[regex] #=> "Idiosyncrätic" |
| 20 | + |
| 21 | +## Keep Expressions |
| 22 | + |
| 23 | +The possible ways to [look around](http://www.regular-expressions.info/lookaround.html) within a regex are: |
| 24 | + |
| 25 | + Syntax | Description | Example |
| 26 | +---------|---------------------|------------------------------- |
| 27 | +`(?=X)` | Positive lookahead | `"Ruby"[/.(?=b)/] #=> "u"` |
| 28 | +`(?!X)` | Negative lookahead | `"Ruby"[/.(?!u)/] #=> "u"` |
| 29 | +`(?<=X)` | Positive lookbehind | `"Ruby"[/(?<=u)./] #=> "b"` |
| 30 | +`(?!X)` | Negative lookbehind | `"Ruby"[/(?<!R|^)./] #=> "b"` |
| 31 | + |
| 32 | +But Ruby also has an additional shortcut syntax to do *positive lookbehinds* via `\K`: |
| 33 | + |
| 34 | + "Ruby"[/Ru\Kby/] #=> "by" |
| 35 | + "Ruby"[/ru\Kby/] #=> nil |
| 36 | + |
| 37 | +## Character Class Intersections |
| 38 | + |
| 39 | +You can nest character classes and AND-connect them with `&&`. Matching all non-vowels here: |
| 40 | + |
| 41 | + "Idiosyncratic".scan /[[a-z]&&[^aeiou]]+/ |
| 42 | + # => ["d", "syncr", "t", "c"] |
| 43 | + |
| 44 | +## Regex Sub-Expressions |
| 45 | + |
| 46 | +You can recursively apply regex groups again with `\g<ref>`. "ref" stands for a group reference (number or name of a capture group). This is different from back-references (`\1` .. `\9`), which will re-match the already matched string, instead of executing the regex again: |
| 47 | + |
| 48 | + # match any number of sequences of 3 identical chars |
| 49 | + regex = /((.)\2{2})\g<1>*/ |
| 50 | + "aaa"[regex] #=> "aaa" |
| 51 | + "abc"[regex] #=> nil |
| 52 | + "aaab"[regex] #=> "aaa" |
| 53 | + "aaabbb"[regex] #=> "aaabbb" |
| 54 | + "aaabbbc"[regex] #=> "aaabbb" |
| 55 | + "aaabbbccc"[regex] #=> "aaabbbccc" |
| 56 | + |
| 57 | +## Match Characters that Belong Together |
| 58 | + |
| 59 | +`\X` treats combined characters as a single character. See [grapheme clusters](http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries) for more information. |
| 60 | + |
| 61 | + string = "R\u{030A}uby" |
| 62 | + string[/./] #=> "R" |
| 63 | + string[/.../] #=> "R̊u" |
| 64 | + string[/\X\X/] #=> "R̊u" |
| 65 | + |
| 66 | +## Relative Back-References |
| 67 | + |
| 68 | +Back-refs can be relatively referenced from the current position via `\k<-n>`: |
| 69 | + |
| 70 | + "Ruby by"[/(R)(u)(by) \k<-1>/] #=> "Ruby by" |
| 71 | + |
| 72 | + |
| 73 | +## Deactivate Backtracking |
| 74 | + |
| 75 | +[Atomic groups](http://www.regular-expressions.info/atomic.html), defined via `(?>X)`, will always try to match the first of all alternatives: |
| 76 | + |
| 77 | + "Rüby"[/R(u*|ü)by/] #=> "Rüby" |
| 78 | + "Rüby"[/R(?>u*|ü)by/] #=> nil |
| 79 | + |
| 80 | +## Turn On Unicode-Matching for `\w`, `\d`, `\s`, and `\b` |
| 81 | + |
| 82 | + "Rüby"[/\w*/] #=> "R" |
| 83 | + "Rüby"[/(?u)\w*/] #=> "Rüby" |
| 84 | + |
| 85 | +## Continue Matching at Last Match Position |
| 86 | + |
| 87 | +When using a method that matches a regex multiple times against a string (like `String#gsub` or `String#scan`), you can reference the position of the last match via `\G`: |
| 88 | + |
| 89 | + "923823723".scan(/\G(.)23/) #=> [["9"], ["8"], ["7"]] |
| 90 | + |
| 91 | +## `String#split` with Capture Groups |
| 92 | + |
| 93 | +The normal way of using `String#split` is this: |
| 94 | + |
| 95 | + "0-0".split(/-/) #=> ["0", "0"] |
| 96 | + |
| 97 | +But if you want to make your code as hard to read as possible, remember that captured groups will be added to the resulting array: |
| 98 | + |
| 99 | + "0-0".split(/(-)/) #=> ["0", "-", "0"] |
| 100 | + "0-0".split(/-(?=(.))/) #=> ["0", "0", "0"] |
| 101 | + "0-0".split(/(((-)))/) #=> ["0", "-", "-", "-", "0"] |
| 102 | + |
| 103 | +## Resources |
| 104 | + |
| 105 | +- [RDoc: Regexp](http://ruby-doc.org/core-2.2.2/Regexp.html) |
| 106 | +- [Onigmo Documentation](https://github.com/k-takata/Onigmo/blob/master/doc/RE) |
0 commit comments