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Regular Expressions 2007


Beginners guide to using Regular Expressions in PHP

Beginners guide to using Regular Expressions in PHP

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  • 1. Regular Expressions for PHP Adding magic to your programming. Geoffrey Dunn (geoff@warmage.com)
  • 2. What are Regular Expressions
    • Regular expressions are a syntax to match text.
    • They date back to mathematical notation made in the 1950s.
    • Became embedded in unix systems through tools like ed and grep.
  • 3. What are RE
    • Perl in particular promoted the use of very complex regular expressions.
    • They are now available in all popular programming languages.
    • They allow much more complex matching than strpos()
  • 4. Why use RE
    • You can use RE to enforce rules on formats like phone numbers, email addresses or URLs.
    • You can use them to find key data within logs, configuration files or webpages.
  • 5. Why use RE
    • They can quickly make replacements that may be complex like finding all email addresses in a page and making them address [AT] site [dot] com.
    • You can make your code really hard to understand
  • 6. Syntax basics
    • The entire regular expression is a sequence of characters between two forward slashes (/)
    • abc - most characters are normal character matches. This is looking for the exact character sequence a, b and then c
    • . - a period will match any character (except a newline but that can change)
    • [abc] - square brackets will match any of the characters inside. Here: a, b or c.
  • 7. Syntax basics
    • ? - marks the previous as optional. so a? means there might be an a
    • (abc)* - parenthesis group patterns and the asterix marks zero or more of the previous character. So this would match an empty string or abcabcabcabc
    • .+ - the backslash is an all purpose escape character. the + marks one or more of the previous character. So this would match ......
  • 8. More syntax tricks
    • [0-4] - match any number from 0 to 4
    • [^0-4] - match anything not the number 0-4
    • swords - match word where there is white space before and after
    • word -  marks a word boundary. This could be white space, new line or end of the string
  • 9. More syntax tricks
    • d{3,12} - d matches any digit ([0-9]) while the braces mark the min and max count of the previous character. In this case 3 to 12 digits
    • [a-z]{8,} - must be at least 8 letters
  • 10. Matching Text
    • Simple check: preg_match(“/^[a-z0-9]+@([a-z0-9]+.)*[a-z0-9]+$/i”, $email_address) > 0
    • Finding: preg_match(“/colou?r:s+([a-zA-Z]+)/”, $text, $matches); echo $matches[1];
    • Find all: preg_match_all(“/<([^>]+)>/”, $html, $tags); echo $tags[2][1];
  • 11. Matching Lines
    • This is more for looking through files but could be for any array of text.
    • $new_lines = preg_grep(“/Jan[a-z]*[s/-](20)?07/”, $old_lines);
    • Or lines that do not match by adding a third parameter of PREG_GREP_INVERT rather than complicating your regular expression into something like /^[^/]|(/[^p])|(/p[^r]) etc...
  • 12. Replacing text
    • preg_replace(
    • “ /[^@]+(@)[a-zA-Z-_d]+(.)[a-zA-Z-_d.]+/”,
    • array(“ [AT] “, “ [dot] “), $post);
  • 13. Splitting text
    • $date_parts = preg_split(“/[-.,/]+/”, $date_string);
  • 14. Tips
    • Comment what your regular expression is doing.
    • Test your regular expression for speed. Some can cause a noticeable slowdown.
    • There are plenty of simple uses like /Width: (d+)/
    • Watch out for greedy expressions. Eg /(<(.+)>)/ will not pull out “b” and “/b” from “<b>test</b>” but instead will pull “b>test</b”. A easy way to change this behaviour is like this: /(<(.+?)>)/
  • 15. References
    • http://en.wikipedia.org/wiki/Regular_expressions
    • http://php.net/manual/en/ref.pcre.php
    • Thank you