Introduction to JavaScript Regular expressions

A regular expression is an object that is used to describe a character pattern. It is used to perform pattern matching (pattern-matching) and "Find-replace" (search-and-replace) tasks in text. The style of JavaScript is similar to that of Perl.

Create

var reg = new RegExp(pattern,modifiers);
Or
var reg = /pattern/modifiers;

Modifiers includes

• G: Performs a global match (finds all matches rather than stops after the first match is found)
  Such as
  ‘Hey, Is this all there Is ‘.replace(/Is/g, ‘is‘);//替换某字符串中所有的Is为is
• I: Perform a case insensitive match
• M: Perform multi-line matching, see here
  PS When you create a new regular, you can have multiple modes, such as/hEllO/ig

The regular correlation method

• . Test (). This method matches a string. and returns TRUE or false based on the matching result. For example

  /e/.test(‘The best things in life are free‘); //true

• . EXEC (). The method will match a string. and returns the first matching item. For example:

  /e/.exec(‘The best things in life are free‘); // ["e"]

In the string method, there is a match method, exec similar to. Such as

‘The best things in life are free‘.match(/e/); //["e"]

Meta-characters in regular

Metacharacters in regular expressions is a special character used to replace a class of characters with the same attributes, which can also be called a character class (character Class).

• . Finds a single character, in addition to line breaks and line terminators.
• \w Find Word characters (letters, numbers, and underscores "_").
• \w Find non-word characters.
• \d find numbers.
• \d to find non-numeric characters.
• \s Find white space characters.
• \s find non-whitespace characters.
• \b matches the word boundary.
• \b Matches a non-word boundary.
• To find the NUL character.
• \ nto find newline characters.
• \f Find a page break.
• \ r to find the carriage return character.
• \ t to find a tab.
• \v Find vertical tabs.
• \xxx finds characters that are specified in octal number XXX.
• \XDD finds the characters specified in hexadecimal number DD.
• \uxxxx finds Unicode characters that are specified in hexadecimal number xxxx.

Curly braces () in regular expressions

To group. Like an email address, we can divide it into

1. @ the front part;
2. @ behind '. ' The previous part;
3. ’.’ The next part;
   If you want to match all Gamil mailboxes in the user name before @ with Jack in the replace with Joel

   ‘[email protected]‘.replace(/^(\w*)(jack)(\w*)@gmail\.com$/, ‘[email protected]‘);  //"iamjo[email protected]"

   This modularity-like idea not only allows us to focus on finding a small part of it at a time, but also allows us to easily replace one of those parts in the back, without having to struggle with the "reaching" pain.

square brackets []

Used to find characters in a range

• [ABC] finds any character between square brackets.
• [^ABC] finds any characters that are not in brackets.
• [0-9] Find any number from 0 to 9.
• [A-z] finds any characters from lowercase a to lower Z.
• [A-z] finds any characters from uppercase A to uppercase Z.
• [A-z] finds any characters from uppercase A to lowercase z.
• [ADGK] finds any character in square brackets.
• [^ADGK] finds any characters that are not in square brackets.

Quantifiers

Match multiple. If a match is matched to 3 numbers available /\d{3}/ .

• n+ matches any string that contains at least one n.
• n matches any string that contains 0 or more N.
• N? Matches any string that contains 0 or one n.
• N{x} matches a string containing a sequence of X N.
• N{x,y} matches a string containing a sequence of X or Y N.
• N{x,} matches a string that contains at least X N of a sequence.
• n$ matches any string that ends with N.
• ^n matches any string that begins with N.
• =n matches any string immediately following the specified string n.
• ?! n matches any string that does not follow the specified string n immediately thereafter.

Exercises

1. Matches the number with a decimal point in the string "1.5 0 123-7-0.4", either positive or negative.
2. Matches a number ([1.5,0,123]) in the string "1.5 0 123", whether it is an integer or a decimal.
3. More Exercises

Advanced greedy and non-greedy modes

The default regular match is greedy mode, that is, the preceding regular matches as many as possible. Such as

/(\d+)(\d+)/.exec(‘12345‘);//结果["12345", "1234", "5"]

Open non-greedy mode, after the quantifier is added?
Such as

/(\d+?)(\d+)/.exec(‘12345‘);//结果["12345", "1", "2345"]

In a more detailed explanation, Dot here

Non-capturing grouping

The contents of the parentheses ?: begin with. Such as:

/(?:\d+)\d+/.exec(‘123‘);// 结果 ["123"]

The

Lookaround is a generic term for forward matching (Lookahead) and backward Matching (lookbehind).
Match forward

Including forward positive matching (Positive Lookahead) and forward negative matching (negative Lookahead), the syntax is ?= and ?! .
Forward match: Matches any string followed by the specified string n.
Forward negative match: matches any string that does not follow the specified string n immediately thereafter.

Similarly, there are backwards matches.
Only Lookahead is currently available in JavaScript, and Lookbehind is not available.

Lookaround Reference Tutorial: http://www.regular-expressions.info/lookaround.html

Regular Expansion Library

Xregexp characteristics

1. Supports all ES5 of the regular syntax.
2. Compatible Explorer 5.5+, Firefox 1.5+, Chrome, Safari, and Opera 11+. can also be used on Nodejs.
3. Higher readability than native.
4. It is easier to use than native.

More Resources

• Regular Concise reference

Reference

• Http://www.w3school.com.cn/js/jsref_obj_regexp.asp
• Http://www.html-js.com/article/A-day-to-learn-JavaScript-JavaScript-regular-expressions-a
• Http://javascript.info/tutorial/regular-expressions-javascript
• Https://github.com/lifesinger/lifesinger.github.com/issues/162#wechat_redirect

Exercise Answer

1. ' 1.5 0 123-7 -0.4 '. Match (/(-?\d+.\d+)/g)
2. ' 1.5 0 123 '. Match (/(\d+.? \d+) |0/g)

recommended Expand Reading

Introduction to JavaScript Regular expressions