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1. By Niko Adrianus Yuwono
BUZOO PHP TEAM
REGULAR EXPRESSIONS LECTURE

2. What is Regular Expressions?
 Regular Expressions or Regex (We’ll mostly use
Regex to call it in this presentation) are a
powerful tool for examining and modifying text.
 Regex use general pattern notation to allow you
describe and parse text.
 PHP supports two different types of regular
expressions: POSIX-extended and Perl-
Compatible Regular Expressions (PCRE). But
we’ll focus on PCRE in this lecture.

3. Delimiters
 When using PCRE functions we need to enclose
the pattern using delimiters.
 Often used delimiters are forward slashes (/),
hash signs (#) and tildes (~ ).
 Example of usage :
 /([^/ | ^-]+).html/
 /</span>(.*?)</span>/

4. Literal-Characters
 Literal characters are normal characters that
match themselves. Alphanumeric characters and
symbols are example of literal characters
 To difference between Meta-Characters and
Literal-Characters we need to add backslash ()
before the literal character to define that
character is a literal character not a meta
character

5. Meta-characters
 Meta-characters are the main power of regular
expressions, with meta-characters it’s possible to
encode alternatives and repetitions in the pattern.
 Meta-characters are divided into two type, meta-
characters outside class, and meta-characters
inside class.

6. Meta-characters Cont’d
 Here is list of meta-character that can work
outside a class :
 , ^ , $ , . , [ , ] , | , ( , ) , ? , * , + , { , }
 And this is the list of meta-character that work
inside a class :
 , ^ , -

7. Character Classes
 Character classes in Regex started by opening
square bracket ([) and closed by and closing
square bracket (])
 A character class matches a single character in
the subject; the character must be in the set of
characters defined by the class.
 Example :
 [a-z] will match any lowercase letter
 [^A-Z] will match a
ny character that is not a uppercase letter

8. Subpatterns
 Subpatterns are delimited by parentheses (round
brackets), which can be nested.
 Subpatterns can do two things :
1. It localizes a set of alternatives. For example,
the pattern hen(dy|rio|ri) matches one of the
words “hendy", “henrio", or “henri". Without the
parentheses, it would match “hendy", “rio" or the
“ri”.
2. It sets up the subpattern as a capturing
subpattern (as defined above).

9. Subpatterns Cont’d
 For example, if the string “kafji tinggi" is matched
against the pattern ((kafji|niko)
(tinggi|tampan)) the captured substrings are
“kafji tinggi", “kafji", and “tinggi", and are
numbered 1, 2, and 3.
 There are often times we don’t need capturing
functions. In that case we can add "?:“ after the
opening parenthesis.

10. Optional Items
 The question mark makes the preceding token in
the regular expression optional.
 Example : colou?r will match both
colour and color.
 You can also wrap a set of characters in
parenthesis to make them optional.
 Example : Jan(uary)? will match both Jan and
January.

11. Repetition
 There are two repetition characters, star ( * ) and
plus ( + ).
 Star ( * ) character will try to match the preceding
token zero or more times.
 Plus ( + ) character will try to match the preceding
token one or more times
 Example :
 [sS]+ will match any character one or more
 [sS]* will match any character zero or more

12. Limiting Repetition
 Sometimes we need to limit some repetition, to
achieve that we can use { } bracket.
 The syntax is {min,max} where min is a must and
you can empty the max but it’ll be counted as
infinity, and if you omit both the coma and max it’ll
repeat the token exactly min times.
 Example :
 ([A-Z]{3}|[0-9]{4}) will matches three letters or four
numbers

13. Greediness
 Greediness is a condition where the regex given
to option try to match the pattern or not to match
the pattern.
 But the regex will always try to match the pattern.
It can cause some trouble to us and will return an
unexpected result.
 For example the regex Feb 23(rd)? to the
string Today is Feb 23rd, 2003, the match will
always be Feb 23rd and not Feb 23.

14. Greediness Cont’d
 Example for repetition :
 You want to get HTML tag for crawling a website.
Usually new people will use <.+> to match the
HTML tag. But it will return a different result than
you expected. Let’s try to match that pattern with
this string -> “Saya <b>suka</b> makan”
 The result will be <b>suka</b>
 Why?

15. Greediness Cont’d
 That’s because of greediness, the pattern <.+>
will try to match dot ( . ) as many as possible.
 Let’s try to do it step by step.
 First the regex will try to search < from this string
“Saya <b>suka</b> makan” so Saya will be
skipped.
 Then after finding < it’ll try to run (.+) that means
to find any character one or more so it’ll read from
b until the end of string. Then it’ll backtracking
until the last > character that have been found so
the result will be <b>suka</b> not <b> and </b>

16. Laziness
 How to fix greediness problem? You can use
laziness by adding ? Question mark after the
repetition or question mark to make them lazy
 But there is also another alternative to laziness
that is negated character class.
 Example for previous question :
 <[^>]+> will match anything except > character