ch4 strings regexp

8/2/2019 CH4 Strings Regexp

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Strings and RegularExpressions in PHP


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January 18, 2005 UPHPU - Mac Newbold 2

String Syntax

Single quotes: a string

No variable interpolation, \ is only escape code

Double quotes: a $better string\nVariables work, standard escape codes work

Here-doc syntax: $foo =


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String Operators

Array-like character access:

$str = MyBigString => $str{3} == B

Concatenation: the dot operator This lets you join strings into . bigger ones

Note: Avoiding embedded newlines in strings thatwrap onto multiple lines is a good idea

Concatenating Assignment : .= $str = My name is; $str .= Mac.\n;


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Variables in Strings

Simple string with a $var in it\n

You can use $an_array[$var] too\n

Sometimes you need ${curl}ies to markwhere the {$var}iable ends

Curlies help on {$big[fancy][$stuff]} too

Where its confusing to embed .$big[ugly][$var].iables, break it up asneeded with concatenation.


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Must-Have String Functions

www.php.net/strings

echo/print(print $foo)==1, echo can,$take,more than one,argument;

Echo shortcut:

trim, ltrim, rtrim/chop remove whitespace

explode, implode/join $arr = explode( , List of words);

$str = implode(,,$arr);
http://www.php.net/stringshttp://www.php.net/strings


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Obligatory C-like Functions

All your old favorites are in there:

printf, sprintf, sscanf, fprintf

strcmp, strlen, strpos, strtok

They all do just what you expect, thoughmany of them have easier alternatives

Gotcha: Some of them (like strpos andfriends) return boolean false, because 0 is avalid result. Always use ===false.


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Basic String Manipulation

Any of this can be done with regularexpressions as well

and in more complex cases, can only be donewith regular expressions

But regular expressions are slower (more later)

str_replace(bar,baz,foobar);

str_repeat(1234567890,8);


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Formatting functions

strtolower, strtoupper

ucfirst, ucwords uppercase first char, orfirst char of each word

wordwrap wrap text to a given width

str_pad(tooshort,15, );

vprintf, vfprintf, vsprintf formatted output number_format add thousands grouping

money_format format as currency


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Special-Purpose Functions

One of PHPs strengths is the way it catersto the common things people need

Many string functions are specifically for usewith things like dates/times, URLs, HTML,and SQL databases

Advice: When you need them, use them.Rolling your own doesnt usually work outthe way you plan it.


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Now for the fun stuff

Regular Expressions

PCRE POSIX

Performance/Speed considerations

Grab bag of cool string functions


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Regular Expressions

Extremely powerful tool for patternmatching same thing used by compilersand interpreters to run your programs

Two flavors in PHP:

PCRE Perl-Compatible Regular Expressions

POSIX Extended

PCRE Advantages multiple languages,more features, faster, and binary-safe


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Basics of REs

They match patterns the magic is in thepattern you tell them to match

They have to be precise, including andexcluding exactly what you want

People get scared of them because thedetails can be tricky

But theyre one of the best tools you havefor doing some pretty fancy string stuff


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RE Patterns

Start with strings and grouping: abc(def)Add alternative branches: abc(def|123)

Wildcard: . matches any char but \n

Quantifiers/Repeating: * = 0 or more, + = 1 or more, ? = 0 or 1

{n} = n times, {n,m} = n to m times

(abc)+(def|123)*(.{2})*At least one abc, maybe some triplets, then an

even number of characters


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Character Classes and Types

[] makes character classes

List of characters and ranges: [a-zA-Z0-9] If you want to use -, put it at the beginning

Escape any special chars with \ as usual

If first char is ^, class is negated

\d = [0-9], \D = [^0-9]

\s = whitespace, \S = non-whitespace \w = [a-zA-Z0-9_], \W = [^a-zA-Z0-9_]

\b = word boundaryzero-width assertion


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Anchors

What if you want to force it to match only atthe beginning of the string? Or to match theentire string?

Use an anchor!

^ as the first char anchors the beginning

$ as the last char anchors the end

(Varies slightly in multi-line mode)


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Greediness and Modifiers

Regular Expressions are Greedy

Theyll keep eating characters as long as theycan keep matching.

Consider: vs. ]*> when matchingagainst Hi

PCRE has modifiers: //

/i = case insensitive/U = un-greedy

/m = multi-line


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Back References

Most commonly used in replace operations,but can be used in match patterns as well

Parentheses not only group, but capture too

Use \ followed by the number of the capture

ab(.)\1(.)\2 will match abccdd or abxxyy,

but not abcccd or abdcdc Can get tricky to count which backref goes

where with nested parentheses


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Modifiers for Parentheses

PCRE Only makes some things possiblethat otherwise couldnt be done

Non-capturing grouping: (?: )

Can simplify back-reference counting

Look-ahead Assertions:

They dont advance the matching position

Positive: (?= ), or Negative: (?! )

Very powerful, but not always easy tounderstand. Trial and error can be your friend!


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PCRE Specifics

www.php.net/pcre

preg_match, preg_match_all, preg_replace,preg_split, preg_grep (filter an array)

Perl REs have a delimiter, usually /, but canbe anything:

preg_match(/foo/,$bar);

preg_match(%/usr/local/bin/%,$path);
http://www.php.net/pcrehttp://www.php.net/pcre


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POSIX Specifics

www.php.net/regex

ereg, ereg_replace, split, eregi, spliti, etc.

[Only] Advantage over PCRE: It doesntrequire the PCRE library to be installed, soits always there in any PHP installation

Other regex engines support thisspecification, though the Perl style seems tobe more popular.
http://www.php.net/regexhttp://www.php.net/regex


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Almost there

Intro to Strings in PHP

(Feel free to tell me how fast or slow to go)

Functions relating to HTML, SQL, etc.

Regular Expressions

PCRE

POSIX

Performance/Speed considerations

Grab bag of cool string functions


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Performance/Speed

Rule of thumb: use the simplest functionthat will get the job done right

strpos instead of substr

str_replace instead of preg_replace

And so forth

The PHP manual online usually includes notes

about speed differences

PCRE is faster than POSIX Regex


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Grab Bag

md5, md5_file Calculate md5 hashes

Great for passwords in databases, etc.

levenshtein, similar_text calculate thesimilarity of two strings

metaphone, soundex calculate how similartwo strings sound when spoken out loud

str_rot13 Encryption algorithm

Protected by the DMCA


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Grab Bag 2

str_shuffle words are much more fun oncetheyve been randomized

count_chars, str_word_count statisticsabout your strings

str_revif it doesnt make sense forward,try it backwards

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