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How to understand JavaScript regular expression, I believe that many inexperienced people do not know what to do. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

I. regular expression creation

1.var reg = / test/

2.var reg = new RegExp ('test')

3. Exec method of regular expression

Var reg = / test/

Var str = 'testrjf'

Var res = reg.exec (str)

/ / will match the test content in str. If there is a match, the matching content will be returned. No null will be returned.

Console.log (res); / / save test as an array

Second, regular expression pattern

Write a method here to facilitate later demonstration.

Function execReg (reg,str) {

Var result = reg.exec (str)

Console.log (result)

} 1. One piece, two tablets, three or four tablets, there are no {n}, {1,}, {1,}.

Reg = / c {1} /; / / matches a c

Str='cainiao'

ExecReg (reg,str); / / returns c

Reg = / c {2} /; / / matches two consecutive c, {n}

Str1='ccaniao'

Str2='cacniao'

ExecReg (reg,str1); / / returns cc

ExecReg (reg,str2); / / returns null. There are no two consecutive cc

Reg = / c {3pr 4} /; / / match 3 or 4 consecutive c

Str1='cccTest'

Str2='ccccTest'

ExecReg (reg,str1); / / returns ccc

ExecReg (reg,str2); / / returns cccc. (regular will match as many as possible)

Reg = / c {1,} /; / / matches more than one c. {2,} {3,}... The same principle

Str1='cccTest'

Str2='chjjk'

ExecReg (reg,str1); / / returns ccc

ExecReg (reg,str2); / / returns c

Reg='/c*/ is equivalent to reg='/c {0,} /

Reg='/c+/ is equivalent to reg='/c {1,} /

Reg='/c?/ is equivalent to reg='/c {0jue 1} /

As a hint here, the regular expression will match as many targets as possible, and if there are four, it will not return three.

2. / ^ begins and ends with $/

The beginning of ^

Reg = / ^ c _ bump / ^ means that only the beginning of the string is matched.

Str1=' vitamin c'

Str2='cainiao'

ExecReg (reg,str1); / / the result is null, because the string "forex analyst" does not begin with c

ExecReg (reg,str2); / / return c this time

End $

Reg = / characters at the end of the string matching the character of the string

Str1='cainiao'

Str2=' vitamin c'

ExecReg (reg,str1); / / returns null

ExecReg (reg,str2); / / returns c

3. Point "."

Reg = /. / /'.' Matches all characters in the string except the newline character\ n

Str1='cainiao'

Str2='bainiao'

ExecReg (reg,str1); / / regular match to the character c

ExecReg (reg,str2); / / regular match to the character b

Reg =. + /

Str1='blueidea- Classic Forum is good.'

Str2='bbs.blueidea.com'

Str3='\ ncainiao'

ExecReg (reg,str1)

/ / return 'blueidea- Classic Forum Hao _. All the characters are matched.

ExecReg (reg,str2); / / returns' bbs.blueidea.com'

ExecReg (reg,str3); / / returns null

4. Or "|"

Reg = / b | c /

Str1='blueidea'

Str2='cainiao'

ExecReg (reg,str1); / / returns b

ExecReg (reg,str2); / / returns c

Reg = / ^ b | c. Resume /

Str='cainiao'

ExecReg (reg,str); / / returns cainiao, which is understood as the beginning b | c and then. +

Reg = / ^ b | c. Resume /

Str='bbs.blueidea.com'

ExecReg (reg,str); / / returns b, which is seen here as the beginning of b or c +

5. Parenthesis

Reg = / ^ (b | c). + /

Str='bbs.blueidea.com'

ExecReg (reg,str); / / this is the beginning b | c, then. +

/ / there is an extra ", b" after the result, which is the match of b | c in ().

/ / what we write in parentheses inside a regular expression will be considered a sub-regular expression, and the matching results will be recorded for later use. Let's ignore this feature for the time being.

6. Character set [abc]

Reg = / ^ [abc] /; / / [abc] represents any character in an or b or c

Str='bbs.blueidea.com'

ExecReg (reg,str); / / returns b

Reg = / ^ [a-zA-Z] [a-zA-Z0-9] + /; / / first ^ [a-zA-Z], then [a-zA-Z0-9] +

Str='test'

ExecReg (reg,str); / / returns test

7. Anti-character set [^ abc]

Reg = / [^ abc] /

Str='blueidea'

ExecReg (reg,str); / / returns l (that is, the first b does not match)

That is, [^ 0-9] means non-numeric, and [^ amurz] represents non-lowercase letters.

8. Boundary and non-boundary

\ b the meaning of the boundary

Reg = /\ bc/;// indicates the c at the beginning or end of the string

Str1='cainiao'

Str2=' vitamin c'

ExecReg (reg,str1); / / returns c, the c of the left boundary

ExecReg (reg,str2); / / returns c, the c of the right boundary

\ B means non-boundary

Reg = /\ Bc/;// represents a string except for the c at the beginning (end)

Str='aicniao'

ExecReg (reg,str); / / returns c, the middle c

9. Digital and non-digital

\ d means a number.

Reg = /\ d /

Str='cainiao8'

ExecReg (reg,str); / / returns 8

\ d means non-numeric

Reg = /\ d /

Str='cainiao8'

ExecReg (reg,str); / / returns c

10. Blank

\ f match feed character,\ nmatch line feed character,\ r match carriage return,\ t match table character,\ v match vertical tab character.

\ s matches a single space, which is equivalent to [\ f\ n\ r\ t\ v].

Reg = /\ s.clients /

Str='This is a test String.'

ExecReg (reg,str); / / returns'is a test String.', matches the first space and all subsequent non-newline characters.

\ S represents a non-space character.

Reg = /\ slide /

Str='This is a test String.'

ExecReg (reg,str); / / This, when the first space is encountered, the regular stops matching

11. Word character

\ w represents a word character, which is equivalent to a character set [a-zA-Z0-9 _].

Reg = /\ walled /

Str='.blueidea'

ExecReg (reg,str); / / blueidea

\ W represents a non-word character, equivalent to [^ a-zA-Z0-9 _].

Reg = /\ Ware /

How about str=' in Chinese?'

ExecReg (reg,str); / / 'how about Chinese?'

twelve。 Reverse reference

Reg = / (\ w)\ 1 /

Str1='blueidea'

Str2='bblueidea'

ExecReg (reg,str1); / / returns null

ExecReg (reg,str2); / / bb

/ / the "\ 1" here is called a backreference, which represents what the word regular expression matches in the first parenthesis.

/ / in the above example, the first parenthesis (\ w) matches b, and "\ 1" also means b, so you can't find b in the rest of the string.

13. Do not record the matching results of sub-regular expressions

Using regularities such as (?: pattern) can avoid saving the matching results in parentheses.

Reg = / ^ (?: B | c). + /

Str='bbs.blueidea.com'

ExecReg (reg,str); / / returns bbs.blueidea.com

/ / the parenthesized subexpression mentioned earlier is not saved.

14. Forward pre-check

Form: (? = pattern), that is, the string to match must be followed by pattern!

Reg = / cainiao (? = 8) /; / / it must be followed by 8, but the result does not match 8

Str1='cainiao9'

Str2='cainiao8'

ExecReg (reg,str1); / / returns null

ExecReg (reg,str2); / / returns cainiao

Form: (?! pattern) and? = are just the opposite, requiring that the string cannot be followed by a pattern.

Reg = / cainiao (?! 8) /

Str1='cainiao9'

Str2='cainiao8'

ExecReg (reg,str1); / / returns cainiao

ExecReg (reg,str2); / / returns null

15. Match metacharacter

*, +,? They all have a special meaning in regular expressions, and characters with special functions are called metacharacters. Then we want to play the match with 'crested'. What about this string?

Reg = / c\? /

Str='c?'

ExecReg (reg,str); / / returns' clocked'

Regular expression modifiers

1.I: performs a case-insensitive match.

Var reg = / b /

Var str = 'BBS'

ExecReg (reg,str); / / returns null

Var reg = / bgambi

Var str = 'BBS'

ExecReg (reg,str); / / B

2.g: perform a global match (find all matches instead of stopping after the first match is found)

It will be explained later.

3.m: perform multi-line matching.

Reg = / ^ b /

Str = 'test\ nbbs';// has line feeds

ExecReg (reg,str); / / null

Reg = / ^ baggage

Str = 'test\ nbbs'

ExecReg (reg,str); / / returns b, starting with the second line b after the line breaks

4. Regular expression method

1.exec method

What the exec method returns is not a matching result string, but an object, which can be confirmed by simply modifying the execReg function to do an experiment.

Function execReg (reg,str) {

Var result = reg.exec (str)

Console.log (typeof result)

Reg = / b /

Str='bbs.bblueidea.com'

ExecReg (reg,str)

/ / shows that the type of result is object. And it's an array-like object.

Where attribute: index input 0. Where index is the index that represents the match in the original string; input is the string that represents the input; and 0 indicates that there is only one matching result, which can be referenced by subscript 0, and this number may change. We can know the total number of matches by returning the length property of the value.

Then modify the execReg function

Function execReg (reg,str) {

Var result = reg.exec (str)

Console.log ('index:'+result.index+'\ n` result\ n')

For (iSuppli)

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