A presentation at ConFoo 2015 in in Montreal, QC, Canada by Drew McLellan
Regular Expressions
For the fearful.
Hello!
flickr.com/photos/85520404@N03/9535499657
Created by Christy Presler from the Noun Project
Humans are great at matching patterns.
RegExp are great at matching patterns.
RegExp Humans
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Regular Expressions Server rewrite rules. Form validation. Text editor search & replace. Application code.
Flavours POSIX basic & extended. Perl and Perl-compatible (PCRE). Most common implementations are Perl-like (PHP, JavaScript and HTML5, mod_rewrite, nginx)
In this exciting episode Basic syntax. Matching. Repeating. Grouping. Replacing.
But fi rst… A regular expression tester is a great way to try things out. There’s an excellent online tester at: regex101.com
RegExp Basics
Basics / regex goes here /
/ regex goes here / modifiers / [A-Z]\w[A-Z] / i Delimiters are usually slashes by default. Some engines allow you to use other delimiters. Modi fi ers include things like case sensitivity.
Basics / this/that / Delimiters and other special characters need to be escaped with backslashes.
Basics / \w\s\d /
Matching
Words \w (lowercase W) / \w / H ello, world, 1234. Matches an alphanumeric character, including underscore.
Global modi fi er The ‘g’ global modi fi er returns all matches. Doesn’t stop at the fi rst match.
Words \w (lowercase W) / \w / g Hello , world , 1234 . Matches an alphanumeric character, including underscore.
Digits \d / \d / Hello, world, 1 234. / \d / g Hello, world, 1234 . Matches single digits 0-9.
Spaces \s / \s / Hello,
world, 1234. / \s / g Hello,
world,
Matches single whitespace character. Includes spaces, tabs, new lines.
Character classes These are all shorthand character classes . Character classes match one character, but o ff er a set of acceptable possibilities for the match. The tokens we’ve looked at a shorthand for more complex character classes.
Words \w [ A-Za-z0-9_ ] Character classes match one character only. They can use ranges like A-Z. They are denoted by [square brackets].
Digits \d [ 0-9 ] Character classes match one character only. They can use ranges like A-Z. They are denoted by [square brackets].
Spaces \s [
\f
] Character classes match one character only. They can use ranges like A-Z. They are denoted by [square brackets]. !!!
Carriage return
New line
Ta b \f Form feed
Custom classes [ ol3 ] / [ ol3 ] /g He llo , w o r l d, 12 3 4. [ a-z0-9- ] / [ a-z0-9- ] /g / 2009 / nice-title
Negative classes [^ ol3 ] / [^ ol3 ] /g He llo , w o r l d, 12 3 4. Use a caret to indicate the class should match none of the given characters. [^ a-z0-9- ] / [^ a-z0-9- ] /g / 2009 / nice-title
Dot A dot (period) matches any character other than a line break. It’s often over-used. Try to use something more speci fi c if possible.
Dot / . /g Hello, world, 1234. Matches any character other than a line break.
!false Developer joke time.
So where does this get us?
Matching Hello world (1980-02-21). / \d\d\d\d-\d\d-\d\d / Hello world ( 1980-02-21 ). So that’s something, right?
Repetition
Repetition Matching single characters gets old fast. There are four main operators or ‘quanti fi ers’ for specifying repetition.
Repetition ? Match zero or once. + Match once or more. * Match zero or more. {x} Match x times. {x,y} Match between x and y times.
Repetition / \d\d\d\d-\d\d-\d\d / / \d {4} -\d {2} -\d {2} / / [ a-z0-9- ]+ /g / 2009 / nice-title
Greediness Repetition quanti fi ers are ‘greedy’ by default. They’ll try to match as many times as possible, within their scope. Sometimes that’s not quite what we want, and we can change this behaviour to make them ‘lazy’.
Greediness / < .+
/ This <em>is</em> some HTML. EXPECTED: This <em> is</em> some HTML. ACTUAL: This <em>is</em> some HTML. Repetition quanti fi ers try to match as many times as they’re allowed to.
Greediness / < .+?
/ This <em> is</em> some HTML. Quanti fi ers can be made ‘lazy’ with a question mark.
Anchors
Anchors Anchors don’t match characters, but the position within the string. There are three main anchors in common use.
Anchors ^ The beginning of the string. $ The end of the string. \b A word boundary.
Anchors / ^ Hello /g Hello , Hello / Hello $ /g Hello, Hello Anchors fi nd matches based on position.
Anchors / cat /g cat con cat enation / \b cat \b /g cat concatenation Word boundaries are useful for avoiding accidental sub- matches.
[‘hip’, ‘hip’] Developer joke time.
Grouping
Grouping Parts of a pattern can be grouped together with (parenthesis). This enables repetition to be applied on the group, and enables us to control how the result is ‘captured’.
? /
? )+ / [ ‘ abc123- ’, ‘ def456- ’, ‘ ghi789 ’ ] Round brackets enable us to create groups that can then be repeated.
? )+ / Groups are captured by default. If you don’t need the group to be captured, make it non- capturing.
Grouping / \w+ @ \w+ . \w+ / drew@allinthehead.com / ( \w+ ) @ ( \w+ . \w+ ) / [ ‘ drew ’, ‘ allinthehead.com ’ ] Capturing groups is very useful! !!!
Grouping / (?<user> \w+ ) @ (?<domain> \w+ . \w+ ) / [ user: ‘ drew ’, domain: ‘ allinthehead.com ’ ] Some engines o ff er named groups.
Replacing
Replacing If you’ve used capturing groups in your pattern, you can re-insert any of those matched values back into your replacement. This is done with ‘back references’. Back references use the index number of the captured group.
Replacing with back references
<?php $str = 'drew@allinthehead.com' ; $pattern = '/(\w+)@(\w+\.\w+)/' ; $replacement = ' $1 is now fred@ $2 ' ; $result = preg_replace ( $pattern , $replacement , $str ); echo $result ; > drew is now fred@allinthehead.com PHP uses the preg (Perl Regular Expression) functions to perform matches and replacements.Replacing with back references var
'drew@allinthehead.com' ; var
/(\w+)@(\w+.\w+)/ ; var
' $1 is now fred@ $2 ’ ; var
str . replace ( pattern , replacement ); console.log ( result );
drew is now fred@allinthehead.com JavaScript uses the replace()
method of a string object.
Putting it to use
HTML5 input validation <input name="sku" type="text" pattern="[A-Z]{3}[0-9]{8-10}"> HTML5 adds the pattern attribute on form fi elds. They’re parsed using the browser’s JavaScript engine.
Apache mod rewrite RewriteEngine On RewriteRule
^news/([1-2]{1}[0-9]{3})/([a-z0-9-]+)/?
/news.php?year=$1&slug=$2 URL rewriting in Apache uses PCRE.
Your application code
<?php $str = 'Look at this https:// www.youtube.com/watch?v=loab4A_SqoQ and this https://www.youtube.com/watch? v=I-19GRsBW-Y' ; $pattern = '/(\w+:\/\/[^\s"]+)/' ; $replacement = '<a href="$1">$1</a>' ; echo preg_replace ( $pattern , $replacement , $str ); > Look at this <a href="https:// www.youtube.com/watch? v=loab4A_SqoQ">https://www.youtube.com/ watch?v=loab4A_SqoQ</a> and this <a href="https://www.youtube.com/watch? v=I-19GRsBW-Y">https://www.youtube.com/ watch?v=I-19GRsBW-Y</a> Don’t copy this example - it’s simpli fi ed and insecure.Further reading
Further reading Teach Yourself Regular Expressions in 10 minutes, by Ben Forta. (Not actually in 10 minutes.) Mastering Regular Expressions, by Je ff rey E. F. Friedl.
Further learning regex101.com
Regular Expressions are a powerful pattern-matching tool for any developer to have at their disposal. From search and replace in a text editor, to web server redirect rules, to string manipulation in your application code, regular expressions are everywhere. This session is an introduction for any developer who feels like they never understood regexp and wants that to change. The code focus will be on PCRE in PHP, but similar for many languages.
The following resources were mentioned during the presentation or are useful additional information.
