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Regular Expressions for Fun and Profit

Regular Expressions for Fun and Profit

Slides for a presentation given at OpenWest 2016 in Sandy, UT.

5f5812aa81bff9a3e1e7a081e76c284c?s=128

Spencer Christensen

July 18, 2016
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    Regular Expressions For Fun And Profit Spencer Christensen | Adobe Analytics SRE
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    Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. - Jamie Zawinski, circa 1997
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    Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems. - Jamie Zawinski, circa 1997 Some people, when confronted with a problem, think “I know, I’ll quote Jamie Zawinski.” Now they have two problems. - Martin Liebach, circa 2009
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    Regular Expressions are...
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    Regular Expressions are...
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    Regular Expressions are...
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    You have been invited to become Regex witches and wizards
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    Patterns and Pattern matching
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    Patterns and Pattern matching
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    Patterns and Pattern matching
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    Describing patterns
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    Describing patterns Using white, cast on 61sts. Mark the centre stitch with a piece of coloured yarn. 1st row: Knit to within 1 st of the centre (on the first row this will be 29 sts and every decrease row after that will be one stitch less), Sl2, K1, PSSO, K to end 2nd row: Knit 3rd row: Using red, knit to within 1 st of the centre, Sl2, K1, PSSO, K to end 4th row: Purl Repeat these four rows, always working rows 1 and 2 in white, and rows 3 and 4 in rainbow stripes. When you have 5sts left work as follows: K1, Sl2, K1, PSSO, K1 Next row: Knit Next row (don't change colours): Sl2, K1, PSSO Fasten off.
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    Describing patterns Poetry and Rhyming patterns Here s an example of ABAB in action, as written ’ by William Shakespeare: A O, if I say, you look upon this verse, B When I, perhaps, compounded am with clay, A Do not so much as my poor name rehearse, B But let your love even with my life decay…
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    Describing patterns Rubik s Cube Notation ’ A single letter by itself means to turn that face clockwise 90 degrees. A letter followed by an apostrophe means to turn that face counterclockwise 90 degrees. A letter with the number 2 after it means to turn that face 180 degrees. e.g. R U R U R U2 R U ’ ’
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    Languages and Symbols using codes to represent ideas and expressions. if (def[d] && def[d].arg && param) { var rw = (d+":"+param).replace(/'|\\/g, '_'); def.__exp = def.__exp || {}; def.__exp[rw] = def[d].text.replace(new RegExp("(^|[^\\w$])" + def[d].arg + "([^\\w$])", "g"), "$1" + param + "$2"); return s + "def.__exp['"+rw+"']"; }
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    Regex as a language matching hello world: /hello world/
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    Regex as a language matching hello world: /hello world/ Limitations of hello world example: • Case sensitive • No explicit start or end of line • Only matches a single space character
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    Regex as a language Special Characters • \ Quote the next metacharacter, or escape • ^ Match the beginning of the line • . Match any character (except newline) • $ Match the end of the string (or before newline at the end of the string) • | Alternation • () Grouping • [ ] Bracketed Character class
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    Regex as a language Quantifiers • * Match 0 or more times • + Match 1 or more times • ? Match 1 or 0 times • {n} Match exactly n times • {n,} Match at least n times • {n,m} Match at least n but not more than m times
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    Examples: /hello +world/ /(h|H)ello +(w|W)orld/ /^(h|H)ello +(w|W)orld$/
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    Quiz time! Write a regex to match any white space at the beginning of a line- zero or more space or tab characters.
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    Quiz time! Write a regex to match any white space at the beginning of a line- zero or more space or tab characters. /^( |\t)*/
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    Character Classes [ ] Square brackets contain possible characters to match one character. • [ABCDEF] matches only the specific literal characters • [A-Z] matches all uppercase letters of the alphabet • [A-Za-z] matches all upper and lower case letters • [0-9] matches all digits • [0-9A-Fa-f] matches hexidecimal numbers, like 9a31f
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    Character Classes Order of contents within a character class doesn't matter as long as the matching is equivalent [abcd] == [dcba] However ranges do matter- [a-Z] != [a-zA-Z]
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    Character Classes Special characters within character class • Invert character class [^a-z], carrot at beginning • Dot, pipe, parens, braces, plus, question mark, star, caret, dollar are literals within a character class • no need to escape, although escaping makes it clear [.|(){}+?*^$] [\.\|\(\)\{\}\+\?\*\^\$] • To get a literal dash, have it at the beginning or escape it [-asdf] or [asdf\-]
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    Escaping characters When desiring a literal non-alphanumeric character and in doubt if you should escape it, then escape it. /USD$[0-9]+\.[0-9]{2}/ /USD\$[0-9]+\.[0-9]{2}/ Double backslash to get a literal backslash character /\\/
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    Quiz time! Write a regex to match an IP address. ei. 10.9.200.12 Hint: use the { } quantifier
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    Quiz time! Write a regex to match an IP address. ei. 10.9.200.12 Hint: use the { } quantifier /[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}/ /([0-9]{1,3}\.?){4}/
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    PCRE character classes as metacharacters Metacharacters or escaped character \w – word character == [a-zA-Z_] \d – digit == [0-9] \s – white space == [ \t\r\n] \t – tab \n – newline \r – carriage return \b – word boundary \x0234 – hex value Inverses: \W == [^\w] == [^a-zA-Z_] \D == [^\d] == [^0-9] \S == [^\s] == [^ \t\r\n]
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    POSIX character classes POSIX character classes are named classes in the form [[:class:]] alpha Any alphabetical character ("[A-Za-z]"). [[:alpha:]] alnum Any alphanumeric character ("[A-Za-z0-9]"). ascii Any character in the ASCII character set. blank A GNU extension, equal to a space or a horizontal tab ("\t"). cntrl Any control character. digit Any decimal digit ("[0-9]"), equivalent to "\d". graph Any printable character, excluding a space. lower Any lowercase character ("[a-z]"). print Any printable character, including a space. punct Any graphical character excluding "word" characters. space Any whitespace character. "\s" including the vertical tab ("\cK"). upper Any uppercase character ("[A-Z]"). word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w". xdigit Any hexadecimal digit ("[0-9a-fA-F]").
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    Quiz time! Write a regex to match any white space at the beginning of a line- zero or more space or tab characters. /^( |\t)*/ => /^\s*/
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    Quiz time! Write a regex to match an IP address. ei. 10.9.200.12 Hint: use the { } quantifier /[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}/ => /\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/ /([0-9]{1,3}\.?){4}/ => /(\d{1,3}\.?){4}/
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    Subexpressions, groups, and captures Parentheses enclose a subexpression, and the match of just that subexpression is saved in a buffer. These buffers can be referenced and used, sometimes called groups or captures. Example: /”(GET|POST) (http[^”]+)”/
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    Subexpressions, groups, and captures Depending on your programming language you can then use those groups and store them in variables and do something with them. Example in python: matches = re.search(r'”(GET|POST) (http[^”]+)”', request_str) if matches: method = matches,group(1) url = matches.group(2)
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    Subexpressions, groups, and captures Groups can be nested, in which case group numbers are based on the left parentheses Example: /(https?:\/\/([^\/]+)/(.*)(\?.*)?)/ How many groups are there?
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    Subexpressions, groups, and captures Groups can be nested, in which case group numbers are based on the left parentheses Example: /(https?:\/\/([^\/]+)/(.*)(\?.*)?)/ How many groups are there? 4 Group 1 is the entire url Group 2 is the hostname Group 3 is the url path Group 4 is the query string if it exists, and is optional. You will need to check if it exists in your code
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    Subexpressions, groups, and captures Things to be aware of with groups/captures • They have overhead copying text to the saved buffers. So if you don't really need the group you can improve performance slightly by using (?:...) notation. This tells the regex engine to not save the subexpression match in a buffer. Example: /(?:this|that|these)/
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    Subexpressions, groups, and captures You can reference a group within the same regex the groups are matching. To reference a group use \1, \2, \3, etc. Example: /(\w+) \1/
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    Performance concerns • If you are only matching a single literal string, it is faster to use a substring function • Be careful using dynamic regexes inside loops. They are evaluated and compile every time. Static regexes can be optimized in perl with /o foreach my $animal (@zoo) { If ($animal =~ /(?:monkey|ape)/o) { $primate_count++; } }
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    Greedy versus Non-greedy matching The quantifiers + and * are greedy by default. Example: /<a href=”.*”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Greedy versus Non-greedy matching The quantifiers + and * are greedy by default. Example: /<a href=”.*”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Greedy versus Non-greedy matching The quantifiers + and * are greedy by default. Example: /<a href=”.*”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Greedy versus Non-greedy matching The quantifiers + and * are greedy by default. Example: /<a href=”.*”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Greedy versus Non-greedy matching The quantifiers + and * are greedy by default. Example: /<a href=”.*”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Greedy versus Non-greedy matching To make them non-greedy simply add ? To the end, like .+? or .*?. This tells the regex engine to look ahead one character on every match which prevents it from going too far. Example: /<a href=”.*?”>/ with the text: <a href=”/index.html”><span class=”button”>Home</span></a>
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    Regular Expressions are Magic
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    Congratulations! Regex Witches and Wizards!
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    Thanks!