08-regular-expressions

Regular Expressions

_{http://regexpal.com/
http://www.regular-expressions.info/
http://mundogeek.net/archivos/2004/07/29/javascript-expresiones-regulares/
http://javascript.espaciolatino.com/lengjs/jsgram/expregulares.htm
http://www.javascriptkit.com/javatutors/redev.shtml}

Regular expressions allow us to search and manipulate text in a very powerful way.
A regular expression consist in:

• A pattern used to locate texts that fits it
• Modificators (optionals) that indicate how to apply the pattern

In Javascript we have available regular expressions objects that we can create:

• With the constructor function RegExp : new RegExp("j.*t")
• With the literal notation: /j.*t/;

Properties of the RegExp objects

The regular expressions objects have the following properties:

• global: With false (by default) it returns only the first element that is found. With true return all the found elements

• ignoreCase: With true it does the matching sensitive to uppercase (false by default)

• multiline: With true it does the search in several lines (false by default)

• lastIndex: The position in where to begin the search (por defecto a 0)

• source: Contains the regular expression

These properties (except lastIndex) cannot be modified once the object has been created

The 3 first properties represent the modificators of the regular expression:

• g: global
• i: ignoreCase
• m: multiline

>>> var re = new RegExp('j.*t', 'gmi');
undefined
>>> re.global
true
>>> re.global = false;
false
>>> re.global
true
>>> var re = /j.*t/ig;
undefined
>>> re.global
true
>>> re.source
"j.*t"

Methods of the RegExp Objects

The RegExp objects have the following methods:

• test(): Returns true if it finds something and false if it doesn't

• exec(): Return an array of string that match the pattern

>>> /j.*t/.test("Javascript")
false
>>> /j.*t/i.test("Javascript")
true
>>> /s(amp)le/i.exec("Sample text")
["Sample", "amp"]
>>> /a(b+)a/g.exec("_abbba_aba_")
["abbba", "bbb"]

String methods that accept regular expressions

_{http://www.javascriptkit.com/javatutors/re3.shtml}

We have available the following methods of the object String to look for inside of a text by using regular expressions:

• match(): Return an array of occurrences

• search(): Return the position of the first occurrence

• replace(): Allow us to replace the found string by another string

• split(): Accepts a regular expression to split a string in elements of an array

replace()

• If we omit the modificator g we only replace the first occurrence
• We can include in the substitution the found string by using $&
• When the regular expression contain groups we can access to the occurrence of every group with $1, $2, etc...
• When specifying the substitution we can pass a function as a parameter where:
  1. The first parameter is the found string
  2. The last parameter is the string where the search is taking place
  3. The penultimate parameter is the position of the occurrence
  4. The rest of the parameters are the occurrences of each group of the pattern

>>> var s = "HelloJavaScriptWorld"
undefined
>>> s.match(/a/);
["a"]
>>> s.match(/a/g);
["a", "a"]
>>> s.match(/j.*a/i); ["Java"]
>>> s.search(/j.*a/i); 5
>>> s.replace(/[A-Z]/g, '');
"elloavacriptorld"
>>> s.replace(/[A-Z]/, ''); "elloJavaScriptWorld"
>>> s.replace(/[A-Z]/g, "_$&"); "_Hello_Java_Script_World"
>>> s.replace(/([A-Z])/g, "_$1"); "_Hello_Java_Script_World"
>>> "[email protected]".replace(/(.*)@.*/, "$1");
"juanmanuel.garrido"
>>> function replaceCallback(match){return "_" + match.toLowerCase();}
undefined
>>> s.replace(/[A-Z]/g, replaceCallback); "_hello_java_script_world"
>>> var sMail = "[email protected]";
undefined
>>> var rRegExp = /(.*)@(.*)\.(.*)/;
undefined
>>> var fCallback = function () { args = arguments; return args[1] + " de " + args[2].toUpperCase(); }
undefined
>>> sMail.replace( rRegExp, fCallback);
"juanmanuel.garrido de SOFTONIC"
>>> args
["[email protected]", "juanmanuel.garrido",
"softonic", "com", 0, "[email protected]"]
>>> var csv = 'one, two,three ,four'; 
>>> csv.split(',');
["one", " two", "three ", "four"]
>>> csv.split(/\s*,\s*/)
["one", "two", "three", "four"]
>>> "test".replace('t', 'r')
"rest"
>>> "test".replace(new RegExp('t'), 'r')
"rest"
>>> "test".replace(/t/, 'r')
"rest"

Regular Expression Syntax

_{https://en.wikipedia.org/wiki/Regular_expression
http://www.addedbytes.com/cheat-sheets/regular-expressions-cheat-sheet/
http://www.visibone.com/regular-expressions/}

[abc]

Looks for coincidences of the characters in the pattern

>>> "some text".match(/[otx]/g)
["o", "t", "x", "t"]

[a-z]

Looks for coincidences in that characters range

[a-d] is the same than [abcd]
[a-z] looks for all the lowercase characters [a-zA-Z0-9_] looks for all the characters, numbers and the underscore

>>> "Some Text".match(/[a-z]/g)
["o", "m", "e", "e", "x", "t"]
>>> "Some Text".match(/[a-zA-Z]/g)
["S", "o", "m", "e", "T", "e", "x", "t"]

###[^abc]

Returns everything that does NOT match the pattern

>>> "Some Text".match(/[^a-z]/g)
["S", " ", "T"]

a|b

Returns a or b (the bar indicates OR)

>>> "Some Text".match(/t|T/g);
["T", "t"]
>>> "Some Text".match(/t|T|Some/g);
["Some", "T", "t"]

a(?=b)

Returns a only is found followed by b

>>> "Some Text".match(/Some(?=Tex)/g);
null
>>> "Some Text".match(/Some(?= Tex)/g);
["Some"]

a(?!b)

Returns a only is found NOT followed by b

>>> "Some Text".match(/Some(?! Tex)/g);
null
>>> "Some Text".match(/Some(?!Tex)/g);
["Some"]

\

Escape character that are used to find special characters used in the pattern as literals

>>> "R2-D2".match(/[2-3]/g)
["2", "2"]
>>> "R2-D2".match(/[2\-3]/g)
["2", "-", "2"]

\n

 New line

\r

Carriage return (To begin a new line \r\n is used in Windows, \n in Unix and \r in Mac)

\f

New page

\t

Tabulation

\v

Vertical Tabulation

\s

Blank espace or any of the previous 5 sequences

>>> "R2\n D2".match(/\s/g)
["\n", " "]

\S

The opposite of the previous sequence. Returns everything but blank spaces and the 5 escape sequences. The same than [^\s]

>>> "R2\n D2".match(/\S/g)
["R", "2", "D", "2"]

\w

Any letter, number, or underscore. The same than [A-Za-z0-9_]

>>> "Some text!".match(/\w/g)
["S", "o", "m", "e", "t", "e", "x", "t"]

\W

The contrary than \w

>>> "Some text!".match(/\W/g)
[" ", "!"]

\d

Locates a number. The same than [0-9]

>>> "R2-D2 and C-3PO".match(/\d/g)
["2", "2", "3"]

\D

The contrary than \d. It locates non-numerical characters. The same than [^0-9] or [^\d]

>>> "R2-D2 and C-3PO".match(/\D/g)
["R", "-", "D", " ", "a", "n", "d", " ", "C", "-", "P", "O"]

\b

A word "limit" (space, puntuation, hyphen...)

>>> "R2D2 and C-3PO".match(/[RD]2/g)
["R2", "D2"]
>>> "R2D2 and C-3PO".match(/[RD]2\b/g)
["D2"]
>>> "R2-D2 and C-3PO".match(/[RD]2\b/g)
["R2", "D2"]

\B

The contrary than \b

>>> "R2-D2 and C-3PO".match(/[RD]2\B/g)
null
>>> "R2D2 and C-3PO".match(/[RD]2\B/g)
["R2"]

^

Represents the beginning of the string where we're looking for. If we have the modificator m it represents the beginning of every line.

>>> "regular\nregular\nexpression".match(/r/g);
["r", "r", "r", "r", "r"]
>>> "regular\nregular\nexpression".match(/^r/g);
["r"]
>>> "regular\nregular\nexpression".match(/^r/mg);
["r", "r"]

$

Represents the final of the string where we're looking for If we have the modificator m it represents the end of every line.

>>> "regular\nregular\nexpression".match(/r$/g);
null
>>> "regular\nregular\nexpression".match(/r$/mg);
["r", "r"]

.

Represents any character but the new line and the carriage return

>>> "regular".match(/r./g);
["re"]
>>> "regular".match(/r.../g);
["regu"]

*

It matches if the preceding pattern happen 0 or more times /.*/ will return everything, including "nothing" (empty string)

>>> "".match(/.*/)
[""]
>>> "anything".match(/.*/)
["anything"]
>>> "anything".match(/n.*h/)
["nyth"]

?

It matches if the preceding pattern happen 0 or once

>>> "anything".match(/ny?/g)
["ny", "n"]

+

It matches if the preceding pattern happen 1 or more times (at least once)

>>> "anything".match(/ny+/g)
["ny"]
>>> "R2-D2 and C-3PO".match(/[a-z]/gi)
["R", "D", "a", "n", "d", "C", "P", "O"]
>>> "R2-D2 and C-3PO".match(/[a-z]+/gi)
["R", "D", "and", "C", "PO"]

{n}

It matches if the preceding pattern happen exactly n times

>>> "regular expression".match(/s/g)
["s", "s"]
>>> "regular expression".match(/s{2}/g)
["ss"]
>>> "regular expression".match(/\b\w{3}/g)
["reg", "exp"]

{min,max}

It matches if the preceding pattern happen between min and max times max can be omitted (will only have minimum) min cannot be omitted

>>> "doooooooooodle".match(/o/g)
["o", "o", "o", "o", "o", "o", "o", "o", "o", "o"]
>>> "doooooooooodle".match(/o{2}/g)
["oo", "oo", "oo", "oo", "oo"]
>>> "doooooooooodle".match(/o{2,}/g)
["oooooooooo"]
>>> "doooooooooodle".match(/o{2,6}/g)
["oooooo", "oooo"]

(pattern)

When the pattern is between parentheses, is being captured and stored so it can be used in substitutions (patterns capture).
Thse captures are available at $1, $2,... $9

>>> "regular expression".replace(/(r)/g, '$1$1')
"rregularr exprression"
>>> "regular expression".replace(/(r)(e)/g, '$2$1')
"ergular experssion"

{?:pattern}

Non capturable pattern (not available at $1, $2, ...)

>>> "regular expression".replace(/(?:r)(e)/g, '$1$1')
"eegular expeession"