Regex Basics

Regular Expression Basics
PHPNW 2008
Ciarán Walsh

What are regular expressions?
Regular expressions allow matching and manipulation of textual data.
Abbreviated as regex or regexp , or alternatively just “patterns”.

Regular Expression Basics Literals bus Matches a ‘ b ’, followed by a ‘ u ’, followed by an ‘ s ’

Regular Expression Basics Anchors ^ Matches at the beginning of a line $ Matches at the end of a line

Regular Expression Basics Character Classes [abc] Matches one of ‘ a ’, ‘ b ’ or ‘ c ’ [a-c] Same as above (character range) [^abc] Matches one character that is not listed . Matches any single character

Regular Expression Basics Alternation a|b Matches one of ‘ a ’ or ‘ b ’ dog|cat Matches one of “dog” or “cat”

Regular Expression Basics Quantifiers (repetition) {x,y} Matches minimum of x and a maximum of y occurrences; either can be omitted * Matches zero or more occurrences (any amount). Same as {0,} + Matches one or more occurrences. Same as {1,} ? Matches zero or one occurrences. Same as {0,1}

Regular Expression Basics Grouping (…) Groups the contents of the parentheses. Affects alternation and quantifiers. Allows parts of the match to be captured for|backward “ for” or “backward” (for|back)ward “ forward” or “backward”

Regular Expression Basics Delimiters pattern / modifiers / /i Makes match case-insensitive

Performing a Match
Returns number of matches (0 or 1)
$matches will contain captured groups
preg_match (
'/Te(.)f?/i' ,
'text' ,
$ matches
);

Performing a Replacement
Returns string after replacement
Can use backreferences with -9
preg_replace (
'/some(text)/' ,
'1' ,
$ text
)

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]|)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*|(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?:
)?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)*<(?:(?: )?[ ])*(?:@(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)
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31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*
](?:(?: )?[ ])*))*)*:(?:(?: )?[ ])*)?(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:
".[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*))*@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;
.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*>(?:(?: )?[ ])*)|(?
:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)*:(?:(?: )?[ ])*(?:(?:(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(
?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]
))*"(?:(?: )?[ ])*))*@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]
+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*|(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?:
)?[ ])*)*<(?:(?: )?[ ])*(?:@(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?
:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*(?:,@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?:
)?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*)*:(?:(?: )?[ ])*)?(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?
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||(?:(?: )?[ ]))*"(?:(?: )?[ ])*))*@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@
,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*>(?:(?: )?[ ])*)(?:,s*(?:(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.
[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[
])*))*@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[
])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*|(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)*<(
?:(?: )?[ ])*(?:@(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+|
|(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*(?:,@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:
.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*)*:(?:(?: )?[ ])*)?(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+|
|(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[ ]))*"(?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|"(?:[^" ||(?:(?: )?[
]))*"(?:(?: )?[ ])*))*@(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-31]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*)(?:.(?:(?: )?[ ])*(?:[^()<>@,;:amp;quot;.[] 00-
031]+(?:(?:(?: )?[ ])+||(?=[["()<>@,;:amp;quot;.[]]))|[([^[] |)*](?:(?: )?[ ])*))*>(?:(?: )?[ ])*))*)?;s*)
Don’t Use Regular Expressions! Don’t Abuse Regular Expressions! Some people, when confronted with a problem, think “ I know, I'll use regular expressions.” Now they have two problems. — Jamie Zawinski

Testing for a Substring if ( preg_match ( '/foo/' , $ var )) if ( strpos ( $ var , 'foo' ) !== false ) if ( preg_match ( '/foo/i' , $ var )) if ( stripos ( $ var , 'foo' ) !== false )

Validating an Integer
Intention is not immediately obvious
Not efficient
if ( preg_match ( '/ ^ d +$ /' , $ value )) { // $value is a positive integer } Regular Expression

Native C library (fast)
Makes the intention obvious
ctype (Character Type) if ( ctype_digit ( $ value )) { // $value is a positive integer }

Intention is fairly clear
Casting is safe practice
Any invalid values will result in zero
$ casted_value = intval ( $ value ); if ( $ casted_value > 0 ) { // $casted_value is a positive (non-zero) integer } Casting

HTML Parsing

Using Regular Expressions

Using Regular Expressions Postcodes /[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][A-Z]{2}/ IP Addresses @^(d{1,2})/(d{1,2})/(d{4})$@

Constructing Patterns
Writing patterns is a balance between matching what you do want, against not matching what you don’t want.

You don’t need to use /…/ to denote a pattern! /…/ to denote a pattern! preg_match ( '/<s> .+ < / s> .+ /' , $ html ) preg_match ( '@<s> .+ </s> .+ @' , $ html )

Greediness $ html = <<< HTML some text some more text! HTML ; preg_match ( "@(.+)@" , $ html , $ matches ); echo $ matches [ 0 ]; preg_match ( "@(.+?)@" , $ html , $ matches ); echo $ matches [ 0 ];

You can make your pattern readable! preg_match ( '`^(w+)://(?:(.+?):(.+?)@)?(.+?).(w+)$`' , $ s , $ matches ) preg_match ( '` ^ (w+):// # Protocol (?: (.+?) # Username : # : (.+?) # Password @ # @ )? # Username/password are optional (.+?) # Hostname .(w+) # Top-level domain $ `x' , $ s , $ matches );

Extracting Captures preg_match ( '`^ (?P<protocol>w+):// (?: (?P<user>.+?) : (?P<pass>.+?) @ )? (?P<host>.+?) .(?P<tld>w+) $`x' , $ s , $ matches ); Array( [0] => http://foo:bar@baz.example.com [protocol] => http [1] => http [user] => foo [2] => foo [pass] => bar [3] => bar [host] => baz.example [4] => baz.example [tld] => com [5] => com) preg_match ( '`^ (?P<protocol>w+):// (?: (?P<user>.+?) : (?P<pass>.+?) @ )? (?P<host>.+?) .(?P<tld>w+) $`x' , $ s , $ matches );

Variable Data if ( preg_match ( "!> $ value </(?:div|span)>!" , $ text )) $ value = preg_quote ( $ value , '!' );

Performing Logic on Replacements preg_replace ( '/w + /e' , 'strtoupper("")' , 'foo bar baz' )
function upper_case_match ( $ matches ) {
return strtoupper ( $ matches [ 0 ]);
}
preg_replace_callback (
'/w + /' ,
'upper_case_match' ,
'foo bar baz'
)

Testing Tools
RegexBuddy
Reggy
http://rubular.com

Any Questions?

Regex Basics

by Jeremy Coates on Feb 20, 2009

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Regex Basics — Presentation Transcript