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Don't Fear the Regex - CapitalCamp/GovDays 2014
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Don't Fear the Regex - CapitalCamp/GovDays 2014


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Have you been scared off by Klingon-looking one-liners in Perl? Do you resort to writing complicated recursive functions just to parse some HTML? Don't! …

Have you been scared off by Klingon-looking one-liners in Perl? Do you resort to writing complicated recursive functions just to parse some HTML? Don't!

I'll demystify regular expressions and show you how best to do them in PHP. We'll cover the syntax and functions that make PHP a great text-parsing language, and give you the foundation to learn more.

As a bonus, I'll give you two cases people often use as examples for regexes that PHP gives you better native ways to accomplish.

Given at CapitalCamp & GovDays 2014

Published in: Software

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  • 1. Don’t Fear the Regex Sandy Smith - CapitalCamp and GovDays 2014
  • 2. Regex Basics
  • 3. Demystifying Regular Expressions So what are Regular Expressions? ! “...a means for matching strings of text, such as particular characters, words, or patterns of characters.” ! Source: Regular_expression 3
  • 4. Demystifying Regular Expressions A Common Joke Some people, when confronted with a problem, think: 'I know, I'll use regular expressions.' Now they have two problems. But really, it’s not that bad, and Regular Expressions (Regex) are a powerful tool. 4
  • 5. Demystifying Regular Expressions So what are they good at? Regex is good at one thing, and that is to match patterns in strings. You might use this to: • Scrape information off of a webpage • Pull data out of text files • Process/Validate data sent to you by a user - Such as phone or credit card numbers - Usernames or even addresses • Evaluate URLs to process what code to execute - Via a framework bootstrap router, or mod_rewrite 5
  • 6. Demystifying Regular Expressions So what do they not do? You can only match/filter/replace patterns of characters you expect to see, if you get unexpected (or non-standard) input, you won’t be able pull the patterns. 6
  • 7. Demystifying Regular Expressions Can you do this in PHP? Yes! PHP contains a great regular expression library, the Perl-Compatible Regular Expression (PCRE) engine. • Perl was (is?) the gold-standard language for doing text manipulation • Lots of programmers knew its regex syntax. • The first PHP regex engine was slow and used a slightly different syntax • PCRE was created to speed things up and let people use their Perl skillz. • Regexes are also useful in text editors! 7
  • 8. Pattern Matching
  • 9. Demystifying Regular Expressions Delimiters All regex patterns are between delimiters. By custom, because Perl did it, this is / • e.g. '/regex-goes-here/' However, PCRE allows anything other than letters, numbers, whitespace, or a backslash () to be delimiters. •'#regex-goes-here#' Why have delimiters? • All will become clear later. 9
  • 10. Demystifying Regular Expressions Straight Text Syntax The most basic regexes match text like strpos() does: • Take the string "PHP is my language of choice" • /PHP/ will match, but /Ruby/ won't. They start getting powerful when you add the ability to only match text at the beginning or end of a line: • Use ^ to match text at the beginning: - /^PHP/ will match, but /^my/ won't. • Use $ to match text at the end: - /choice$/ will match, but /PHP$/ won't. 10
  • 11. Demystifying Regular Expressions Basic Pattern Matching Regular expressions are often referred to as "pattern matching," because that's what makes them powerful. Use special characters to match patterns of text: . matches any single character: /P.P/ matches PHP or PIP + matches one or more of the previous character: /PH+P/ matches PHP or PHHHP * matches zero or more of the previous characters: /PH*P/ matches PHP or PP or PHHHHP 11
  • 12. Demystifying Regular Expressions Basic Pattern Matching ? matches zero or one of the previous character: /PH?P/ matches PHP or PP but not PHHP {<min>,<max>} matches from min to max occurrences of the previous character: /PH{1,2}P/ matches PHP or PHHP but not PP or PHHHP 12
  • 13. Demystifying Regular Expressions Powerful Basic Patterns You can use combinations of these patterns to find lots of things. Here are the most common: .? Find zero or one characters of any type: /P.?P/ gets you PP, PHP, PIP, but not PHHP. .+ Find one or more characters of any type: /P.+P/ gets you PHP, PIP, PHPPHP, PIIIP, but not PP. .* Find zero or more characters of any type: /P.*P/ gets PP, PHP, PHPHP, PIIIP, but not PHX. 13
  • 14. Demystifying Regular Expressions Beware of Greed .* and .+ are "greedy" by default, meaning they match as much as they can while still fulfilling the pattern. /P.+P/ will match not only "PHP" but "PHP PHP" Greedy pattern don't care. What if you want to only match "PHP", "PHHP", or "PIP", but not "PHP PHP"? ? kills greed. /P.*?P/ will match PHP, PP, or PIIIP but only the first PHP in "PHP PHP" Great for matching tags in HTML, e.g. /<.+?>/ 14
  • 15. Demystifying Regular Expressions Matching literal symbols If you need to match a character used as a symbol, such as $, +, ., ^, or *, escape it by preceding it with a backslash (). /./ matches a literal period (.). /^$/ matches a dollar sign at the beginning of a string. 15
  • 16. Demystifying Regular Expressions Calling this in PHP To match regular expressions in PHP, use preg_match(). It returns 1 if a pattern is matched and 0 if not. It returns false if you blew your regex syntax. Simplest Example: 16 $subject = "PHP regex gives PHP PEP!"; $found = preg_match("/P.P/", $subject); echo $found; // returns 1
  • 17. Character Classes and Subpatterns
  • 18. Demystifying Regular Expressions Character Classes Matching any character can be powerful, but lots of times you'll want to only match specific characters. Enter character classes. • Character classes are enclosed by [ and ] (square brackets) • Character classes can be individual characters • They can also be ranges • They can be any combination of the above • No "glue" character: any character is a valid pattern 18
  • 19. Demystifying Regular Expressions Character Class Examples Single character: [aqT,] matches a, q,T (note the case), or a comma (,) Range: [a-c] matches either a, b, or c (but not A or d or ...) [4-6] matches 4, 5, or 6 Combination [a-c4z6-8] matches a, b, c, 4, z, 6, 7, or 8 19
  • 20. Demystifying Regular Expressions Negative classes Even more powerful is the ability to match anything except characters in a character class. • Negative classes are denoted by ^ at the beginning of the class • [^a] matches any character except a • [^a-c] matches anything except a, b, or c • [^,0-9] matches anything except commas or digits 20
  • 21. Demystifying Regular Expressions Using Character Classes Just using the elements you've learned so far, you can write the majority of patterns commonly used in regular expressions. /<[^>]+?/>/ matches all the text inside an HTML tag /^[0-9]+/ matches the same digits PHP will when casting a string to an integer. /^[a-zA-Z0-9]+$/ matches a username that must be only alphanumeric characters /^$[a-zA-Z_][a-zA-Z0-9_]*$/ matches a valid variable name in PHP. 21
  • 22. Demystifying Regular Expressions Subpatterns What if you want to look for a pattern within a pattern? Or a specific sequence of characters? It's pattern inception with subpatterns. • Subpatterns are enclosed by ( and ) (parentheses) • They can contain a string of characters to match as a group, such as (cat) • Combined with other symbols, this means you can look for catcatcat with (cat)+ • You can look for alternate strings, such as (cat|dog) matching cat or dog • They can also contain character classes and expressions 22
  • 23. Demystifying Regular Expressions Revisiting preg_match() What if you want to extract the text that's been matched? • preg_match() has an optional third argument for an array that it will fill with the matched results. • Why an array? Because it assumes you'll be using subpatterns. • The first element of the array is the text matched by your entire pattern. • The second element is the text matched by the first subpattern (from left to right), the second with the second, and so on. • The array is passed by reference for extra confusion. 23
  • 24. Demystifying Regular Expressions Matching with Subpatterns <?php $variable = '$variable'; $pattern = ‘/^$([a-zA-Z_][a-zA-Z_0-9]*)$/'; $matches = array(); $result = preg_match($pattern, $variable, $matches); var_dump($matches); // passed by reference /* array(2) { [0]=> string(9) "$variable" [1]=> string(8) "variable" } */ 24
  • 25. Demystifying Regular Expressions Alternatives Subpatterns can do more than simply group patterns for back references.They can also let you identify strings of alternatives that you can match, using the pipe character (|) to separate them. For example, /(cat|dog)/ will match cat or dog. When combined with other patterns, it becomes powerful: /^((http|https|ftp|gopher|file)://)?([^.]+?)/ would let you match the first domain or subdomain of a URL. 25
  • 26. Demystifying Regular Expressions Escape Sequences Now that we've made you write [0-9] a whole bunch of times, let's show you a shortcut for that plus a bunch of others. (Ain't we a pill?) • d gets you any digit. D gets you anything that isn't a digit. • s gets you any whitespace character. Careful, this usually* includes newlines (n) and carriage returns (r). S gets you anything not whitespace. 26
  • 27. Demystifying Regular Expressions Escape Sequences (cont'd) You've already seen how to escape special characters used in regular expressions as well as replacements for character classes.What about specific whitespace characters? • t is a tab character. • n is the Unix newline (line feed); also the default line ending in PHP. • r is the carriage return. Formerly used on Macs. rn is Windows's end of line statement; R gets you all three. • h gets you any non-line ending whitespace character (horizontal whitespace). 27
  • 28. Demystifying Regular Expressions Special Escape Sequences There are some oddities that are holdovers from the way Perl thinks about regular expressions. • w gets you a "word" character, which means
 [a-zA-Z0-9_] (just like a variable!), but is locale-aware (captures accents in other languages). W is everything else. I'm sure Larry Wall has a long-winded explanation. Note it doesn't include a hyphen (-) or apostrophe ('). • b is even weirder. It's a "word boundary," so not a character, per se, but marking the transition between whitespace and "word" characters as defined by w. 28
  • 29. Demystifying Regular Expressions Back References Rather than repeat complicated subpatterns, you can use a back reference. Each back reference is denoted by a backslash and the ordinal number of the subpattern. (e.g., 1, 2, 3, etc.) As in preg_match(), subpatterns count left parentheses from left to right. • In /(outersub(innersub))12/, 1 matches outersub(innersub), and 2 matches innersub. • Similarly, in /(sub1)(sub2)12/, 1 matches sub1, and 2 matches sub2. 29
  • 30. Demystifying Regular Expressions Back Reference Example The easiest real-world example of a back reference is matching closing to opening quotes, whether they are single or double. 30 $subject = 'Get me "stuff in quotes."'; $pattern = '/(['"])(.*?)1/'; $matches = array(); $result = preg_match($pattern, $subject, $matches); var_dump($matches); /* array(3) { [0]=> string(18) ""stuff in quotes."" [1]=> string(1) """ [2]=> string(16) "stuff in quotes." } */
  • 31. Demystifying Regular Expressions Replacing Matching is great, but what about manipulating strings? Enter preg_replace(). Instead of having a results array passed by reference, preg_replace returns the altered string. It can also work with arrays. If you supply an array, it performs the replace on each element of the array and returns an altered array. 31
  • 32. Demystifying Regular Expressions preg_replace() <?php $pattern = '/(['"]).*?1/'; $subject = 'Look at the "stuff in quotes!"'; $replacement = '$1quoted stuff!$1'; $result = preg_replace($pattern, $replacement, $subject); echo $result; // Look at the "quoted stuff!" ! $pattern = array('/quick/', '/brown/', '/fox/'); $subject = array('overweight programmer', 'quick brown fox', 'spry red fox'); $replacement = array('slow', 'black', 'bear'); $result = preg_replace($pattern, $replacement, $subject); var_dump($result); /* array(3) { [0]=> string(21) "overweight programmer" [1]=> string(15) "slow black bear" [2]=> string(13) "spry red bear" } */ 32
  • 33. Demystifying Regular Expressions Case-insensitive modifier Remember when we said we’d explain why regular expressions use delimiters? By now, some of you may have asked about case-sensitivity, too, and we said we’d get to it later. Now is the time for both. Regular expressions can have options that modify the behavior of the whole expression.These are placed after the expression, outside the delimiters. Simplest example: i means the expression is case- insensitive. /asdf/i matches ASDF, aSDf, and asdf. 33
  • 34. Demystifying Regular Expressions When not to use Regex? One more important topic. Regular expressions are powerful, but when abused, they can lead to harder-to- maintain code, security vulnerabilities, and other bad things. In particular, don’t reinvent the wheel. PHP already has great, tested libraries for filtering and validating input input (filter_var) and parsing URLs (parse_url). Use them. The rules for valid email addresses are surprisingly vague, so best practice is to simply look for an @ or use filter_var’s FILTER_VALIDATE_EMAIL and try to send an email to the supplied address with a confirmation link. 34
  • 35. Demystifying Regular Expressions Thank you! There’s much more to learn! Follow us on Twitter: @phparch @SandyS1 - Me Feedback is always welcome: 35