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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.

Spencer Christensen

July 18, 2016
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  1. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regular Expressions For Fun And Profit
    Spencer Christensen | Adobe Analytics SRE

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  2. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  3. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  4. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regular Expressions are...

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  5. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regular Expressions are...

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  6. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regular Expressions are...

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  7. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    You have been invited to become Regex witches and wizards

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  8. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Patterns and Pattern matching

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  9. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Patterns and Pattern matching

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  10. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Patterns and Pattern matching

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  11. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Describing patterns

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  12. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  13. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  14. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  15. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  16. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regex as a language
    matching hello world:
    /hello world/

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  17. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  18. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  19. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  20. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Examples:
    /hello +world/
    /(h|H)ello +(w|W)orld/
    /^(h|H)ello +(w|W)orld$/

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  21. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  22. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  23. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  24. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  25. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  26. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  27. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Quiz time!
    Write a regex to match an IP address.
    ei. 10.9.200.12
    Hint: use the { } quantifier

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  28. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  29. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  30. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  31. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  32. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  33. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  34. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  35. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  36. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  37. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  38. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  39. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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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  40. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Greedy versus Non-greedy matching
    The quantifiers + and * are greedy by default.
    Example: //
    with the text:
    class=”button”>Home

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  41. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Greedy versus Non-greedy matching
    The quantifiers + and * are greedy by default.
    Example: //
    with the text:
    class=”button”>Home

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  42. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Greedy versus Non-greedy matching
    The quantifiers + and * are greedy by default.
    Example: //
    with the text:
    class=”button”>Home

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  43. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Greedy versus Non-greedy matching
    The quantifiers + and * are greedy by default.
    Example: //
    with the text:
    class=”button”>Home

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  44. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Greedy versus Non-greedy matching
    The quantifiers + and * are greedy by default.
    Example: //
    with the text:
    class=”button”>Home

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  45. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    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: //
    with the text:
    class=”button”>Home

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  46. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Regular Expressions are Magic

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  47. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Congratulations!
    Regex Witches and Wizards!

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  48. © 2015 Adobe Systems Incorporated. All Rights Reserved. Adobe Confidential.
    Thanks!

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