regular-expressions lecture 28-string regular expression

regular-expressions lecture 28-string regular expression

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  CSE 341 Lecture 28 Regularexpressions slides created by Marty Stepp http://www.cs.washington.edu/341/
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  2 Influences on JavaScript •Java: basic syntax, many type/method names • Scheme: first-class functions, closures, dynamism • Self: prototypal inheritance • Perl: regular expressions • Historic note: Perl was a horribly flawed and very useful scripting language, based on Unix shell scripting and C, that helped lead to many other better languages.  PHP, Python, Ruby, Lua, ...  Perl was excellent for string/file/text processing because it built regular expressions directly into the language as a first- class data type. JavaScript wisely stole this idea.
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  3 What is aregular expression? /[a-zA-Z_-]+@(([a-zA-Z_-])+.)+[a-zA-Z]{2,4}/ • regular expression ("regex"): describes a pattern of text  can test whether a string matches the expr's pattern  can use a regex to search/replace characters in a string  very powerful, but tough to read • regular expressions occur in many places:  text editors (TextPad) allow regexes in search/replace  languages: JavaScript; Java Scanner, String split  Unix/Linux/Mac shell commands (grep, sed, find, etc.)
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  4 String regexp methods .match(regexp)returns first match for this string against the given regular expression; if global /g flag is used, returns array of all matches .replace(regexp, text) replaces first occurrence of the regular expression with the given text; if global /g flag is used, replaces all occurrences .search(regexp) returns first index where the given regular expression occurs .split(delimiter[,limit ]) breaks apart a string into an array of strings using the given regular as the delimiter; returns the array of tokens
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  5 Basic regexes /abc/ • aregular expression literal in JS is written /pattern/ • the simplest regexes simply match a given substring • the above regex matches any line containing "abc"  YES : "abc", "abcdef", "defabc", ".=.abc.=."  NO : "fedcba", "ab c", "AbC", "Bash", ...
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  6 Wildcards and anchors .(a dot) matches any character except n  /.oo.y/ matches "Doocy", "goofy", "LooPy", ...  use . to literally match a dot . character ^ matches the beginning of a line; $ the end  /^if$/ matches lines that consist entirely of if < demands that pattern is the beginning of a word; > demands that pattern is the end of a word  /<for>/ matches lines that contain the word "for"
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  7 String match string.match(regex) • ifstring fits pattern, returns matching text; else null  can be used as a Boolean truthy/falsey test: if (name.match(/[a-z]+/)) { ... } • g after regex for array of global matches  "obama".match(/.a/g) returns ["ba", "ma"] • i after regex for case-insensitive match  name.match(/Marty/i) matches "marty", "MaRtY"
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  8 String replace string.replace(regex, "text") •replaces first occurrence of pattern with the given text  var state = "Mississippi"; state.replace(/s/, "x") returns "Mixsissippi" • g after regex to replace all occurrences  state.replace(/s/g, "x") returns "Mixxixxippi" • returns the modified string as its result; must be stored  state = state.replace(/s/g, "x");
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  9 Special characters | meansOR  /abc|def|g/ matches lines with "abc", "def", or "g"  precedence: ^Subject|Date: vs. ^(Subject|Date):  There's no AND & symbol. Why not? () are for grouping  /(Homer|Marge) Simpson/ matches lines containing "Homer Simpson" or "Marge Simpson" starts an escape sequence  many characters must be escaped: / $ . [ ] ( ) ^ * + ?  ".n" matches lines containing ".n"
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  10 Quantifiers: * +? * means 0 or more occurrences  /abc*/ matches "ab", "abc", "abcc", "abccc", ...  /a(bc)/" matches "a", "abc", "abcbc", "abcbcbc", ...  /a.*a/ matches "aa", "aba", "a8qa", "a!?_a", ... + means 1 or more occurrences  /a(bc)+/ matches "abc", "abcbc", "abcbcbc", ...  /Goo+gle/ matches "Google", "Gooogle", "Goooogle", ... ? means 0 or 1 occurrences  /Martina?/ matches lines with "Martin" or "Martina"  /Dan(iel)?/ matches lines with "Dan" or "Daniel"
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  11 More quantifiers {min,max} meansbetween min and max occurrences  /a(bc){2,4}/ matches lines that contain "abcbc", "abcbcbc", or "abcbcbcbc" • min or max may be omitted to specify any number  {2,} 2 or more  {,6} up to 6  {3} exactly 3
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  12 Character sets [ ]group characters into a character set; will match any single character from the set  /[bcd]art/ matches lines with "bart", "cart", and "dart"  equivalent to /(b|c|d)art/ but shorter • inside [], most modifier keys act as normal characters  /what[.!*?]*/ matches "what", "what.", "what!", "what?**!", ... – Exercise : Match letter grades e.g. A+, B-, D.
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  13 Character ranges • insidea character set, specify a range of chars with -  /[a-z]/ matches any lowercase letter  /[a-zA-Z0-9]/ matches any letter or digit • an initial ^ inside a character set negates it  /[^abcd]/ matches any character but a, b, c, or d • inside a character set, - must be escaped to be matched  /[-+]?[0-9]+/ matches optional - or +, followed by at least one digit – Exercise : Match phone numbers, e.g. 206-685-2181 .
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  14 Built-in character ranges •b word boundary (e.g. spaces between words) • B non-word boundary • d any digit; equivalent to [0-9] • D any non-digit; equivalent to [^0-9] • s any whitespace character; [ fnrtv...] • s any non-whitespace character • w any word character; [A-Za-z0-9_] • W any non-word character • xhh, uhhhh the given hex/Unicode character  /w+s+w+/ matches two space-separated words
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  15 Regex flags /pattern/g global;match/replace all occurrences /pattern/i case-insensitive /pattern/m multi-line mode /pattern/y "sticky" search, starts from a given index • flags can be combined: /abc/gi matches all occurrences of abc, AbC, aBc, ABC, ...
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  16 Back-references • text "captured"in () is given an internal number; use number to refer to it elsewhere in the pattern  0 is the overall pattern,  1 is the first parenthetical capture, 2 the second, ...  Example: "A" surrounded by same character: /(.)A1/  variations – (?:text) match text but don't capture – a(?=b) capture pattern b but only if preceded by a – a(?!b) capture pattern b but only if not preceded by a
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  17 Replacing with back-references •you can use back-references when replacing text:  refer to captures as $number in the replacement string  Example: to swap a last name with a first name: var name = "Durden, Tyler"; name = name.replace(/(w+),s+(w+)/, "$2 $1"); // "Tyler Durden" – Exercise : Reformat phone numbers from 206-685-2181 format to (206) 685.2181 format.
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  18 The RegExp object newRegExp(string) new RegExp(string, flags) • constructs a regex dynamically based on a given string var r = /ab+c/gi; is equivalent to var r = new RegExp("ab+c", "gi");  useful when you don't know regex's pattern until runtime – Example: Prompt user for his/her name, then search for it. – Example: The empty regex (think about it).
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  19 Working with RegExp •in a regex literal, forward slashes must be escaped: /http[s]?://w+.com/ • in a new RegExp object, the pattern is a string, so the usual escapes are necessary (quotes, backslashes, etc.): new RegExp("http[s]?://w+.com") • a RegExp object has various properties/methods:  properties: global, ignoreCase, lastIndex, multiline, source, sticky; methods: exec, test
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  20 Regexes in editorsand tools • Many editors allow regexes in their Find/Replace feature • many command-line Linux/Mac tools support regexes grep -e "[pP]hone.*206[0-9]{7}" contacts.txt