regular expression

from ：https://github.com/ziishaned/learn-regex

What is regular expression ？

A regular expression is a special set of text consisting of letters and symbols , It can be used to find sentences from the text that meet the format you want .

A regular expression is a pattern that matches the body string from left to right .
“Regular expression” This word is rather awkward , We often use abbreviated terms “regex” or “regexp”.
Regular expressions can replace strings in text from a base string according to a certain matching pattern 、 validate form 、 Extract strings and so on .

Imagine you're writing an app , Then you want to set a user naming rule , Let the user name contain the characters 、 Numbers 、 Underscores and hyphens , And limit the number of characters , So that the name doesn't look so ugly .
We use the following regular expression to verify a user name ：

The regular expression above can accept john_doe、jo-hn_doe、john12_as.
But don't match Jo, Because it contains capital letters and it's too short .

Catalog

• 1. Basic match
• 2. Metacharacters
  • 2.1 Dot operator .
  • 2.2 Character set
    • 2.2.1 Negative character set
  • 2.3 Repeat the number
    • 2.3.1 * Number
    • 2.3.2 + Number
    • 2.3.3 ? Number
  • 2.4 {} Number
  • 2.5 (…) Characteristic groups
  • 2.6 | Or operator
  • 2.7 Transcode special characters
  • 2.8 Anchor point
    • 2.8.1 ^ Number
    • 2.8.2 $ Number
• 3. Shorthand character set
• 4. Zero width assertion ( Pre check before and after )
  • 4.1 ?=… Is asserting first
  • 4.2 ?!.. Negative antecedent assertion
  • 4.3 ?<= … Just after the assertion
  • 4.4 ?<!.. Negative post assertion
• 5. sign
  • 5.1 Ignore case （Case Insensitive）
  • 5.2 Global search （Global search）
  • 5.3 Multiline modifier （Multiline）
• Add extra
• contribution
• license

1. Basic match

Regular expression is actually the format of the search , It's a combination of letters and numbers .
for example ： A regular expression the, It means a rule ： By letter t Start , Next is h, And then there is e.

"the" => The fat cat sat on the mat.

Practice online

Regular expressions 123 Match string 123. It's character by character compared to the input regular expression .

Regular expressions are case sensitive , therefore The It doesn't match the.

"The" => The fat cat sat on the mat.

Practice online

2. Metacharacters

Regular expressions mainly depend on metacharacters .
Metacharacters do not represent their literal meaning , They all have a special meaning . Some metacharacters have special meanings when written in square brackets . Here's an introduction to some metacharacters ：

Metacharacters	describe
.	Period matches any single character except line breaks .
[ ]	Character type . Match any character in brackets .
[^ ]	Negative character type . Match any character except in square brackets
*	matching >=0 A repetition is in * The character before the number .
+	matching >=1 A repetition of + The character before the number .
?	Mark ? The previous characters are optional .
{n,m}	matching num The character or character set before braces (n <= num <= m).
(xyz)	Character set , Match with xyz Exactly the same string .
|	Or operator , Match the characters before or after the symbol .
\	Escape character , Used to match some reserved characters [ ] ( ) { } . * + ? ^ $ \ |
^	Match from the start line .
$	Match from the end .

2.1 Dot operator .

. Is the simplest example of metacharacters .
. Match any single character , But don't match newline .
for example , expression .ar Match an arbitrary character followed by a and r String .

".ar" => The car parked in the garage.

Practice online

2.2 Character set

Character sets are also called character classes .
Square brackets are used to specify a character set .
Use hyphens in square brackets to specify the range of the character set .
The character set in square brackets doesn't care about order .
for example , expression [Tt]he matching the and The.

"[Tt]he" => The car parked in the garage.

Practice online

A period in square brackets means a period .
expression ar[.] matching ar. character string

"ar[.]" => A garage is a good place to park a car.

Practice online

2.2.1 Negative character set

Generally speaking ^ Represents the beginning of a string , But it's used at the beginning of a square bracket , It means that this character set is negative .
for example , expression [^c]ar Match one followed by ar Except c Any character of .

"[^c]ar" => The car parked in the garage.

Practice online

2.3 Repeat the number

Followed by metacharacters +,* or ? Of , Used to specify the number of times to match a subpattern .
These metacharacters have different meanings in different situations .

2.3.1 * Number

* Number matches stay * The previous characters appear Greater than or equal to 0 Time .
for example , expression a* matching 0 Or more a Start character . expression [a-z]* Match all strings in a row that start with lowercase letters .

"[a-z]*" => The car parked in the garage #21.

Practice online

* Characters and . Character matching can match all characters .*.
* And symbols that match spaces \s Use it together , Like the expression \s*cat\s* matching 0 Or more spaces and 0 Or more spaces cat character string .

"\s*cat\s*" => The fat cat sat on the concatenation.

Practice online

2.3.2 + Number

+ Number matches + The character before the sign appears >=1 Time .
For example, an expression c.+t Match with initial c Begin with t ending , A string followed by at least one character .

"c.+t" => The fat cat sat on the mat.

Practice online

2.3.3 ? Number

In regular expressions, metacharacters ? The characters marked before the symbol are optional , That is to say 0 or 1 Time .
for example , expression [T]?he Match string he and The.

"[T]he" => The car is parked in the garage.

Practice online

"[T]?he" => The car is parked in the garage.

Practice online

2.4 {} Number

In regular expressions {} It's a quantifier , It is often used to limit the number of times a character or group of characters can be repeated .
for example , expression [0-9]{2,3} Match the least 2 Most bits 3 position 0~9 The number of .

"[0-9]{2,3}" => The number was 9.9997 but we rounded it off to 10.0.

Practice online

We can omit the second parameter .
for example ,[0-9]{2,} Match at least two 0~9 The number of .

"[0-9]{2,}" => The number was 9.9997 but we rounded it off to 10.0.

Practice online

If the comma is also omitted, it means repeating a fixed number of times .
for example ,[0-9]{3} matching 3 Digit number

"[0-9]{3}" => The number was 9.9997 but we rounded it off to 10.0.

Practice online

2.5 (...) Characteristic groups

A feature group is a group written in (...) Sub pattern in .(...) The content contained in will be seen as a whole , And mathematical parentheses （ ） It's the same thing . for example , expression (ab)* Match appears continuously 0 Or more ab. If not used (...) , So the expression ab* The match appears continuously 0 Or more b . Like before {} It is used to indicate the specified number of occurrences of the preceding character . But if {} And then we add the eigenvalue group (...) It means that the characters in the whole group are repeated N Time .

We can still do that () Chinese or character | Represents or . for example ,(c|g|p)ar matching car or gar or par.

"(c|g|p)ar" => The car is parked in the garage.

Practice online

A recent example of how I handle mass mailing , You need to exchange the last name and first name in the name string ：

Xiaodong Chen
Dawei Li
In the east king

stay sublime replace ：

(.{1,2}) (.{1,2})
$2 $1

After replacement ：

Chen Xiaodong
Li Dawei
Wang Dong

2.6 | Or operator

Or operator means or , As a condition of judgment .

for example (T|t)he|car matching (T|t)he or car.

"(T|t)he|car" => The car is parked in the garage.

Practice online

2.7 Transcode special characters

Backslash \ Used in expressions to transcode the characters immediately following . Is used to specify the { } [ ] / \ + * . $ ^ | ? These special characters . If you want to match these special characters, you need to precede them with a backslash \.

for example . It is used to match all characters except line breaks . If you want to match . It should be written as \. Here's an example \.? It's a selective match .

"(f|c|m)at\.?" => The fat cat sat on the mat.

Practice online

2.8 Anchor point

In regular expressions , To match a string at the beginning or the end of a specified string, you need to use an anchor .^ Specify the beginning ,$ Specify the end .

2.8.1 ^ Number

^ Used to check whether the matching string is at the beginning of the matched string .

for example , stay abc Using expressions in ^a You'll get the results a. But if ^b Will not match any results . Because in string abc It's not because of b start .

for example ,^(T|t)he Match with The or the Starting string .

"(T|t)he" => The car is parked in the garage.

Practice online

"^(T|t)he" => The car is parked in the garage.

Practice online

example ： It can be used “^23” Match the following lines with 23 Starting string

123
23
+233
a23

2.8.2 $ Number

In the same way in ^ Number ,$ The number is used to match whether the character is the last .

for example ,(at\.)$ Match with at. a null-terminated string .

"(at\.)" => The fat cat. sat. on the mat.

Practice online

"(at\.)$" => The fat cat. sat. on the mat.

Practice online

3. Shorthand character set

Regular expressions provide some common shorthand for character sets . as follows :

Abbreviation	describe
.	All characters except line breaks
\w	Match all alphanumeric , Equate to [a-zA-Z0-9_]
\W	Match all non alphanumeric , That's the symbol , Equate to ： [^\w]
\d	Match the Numbers ： [0-9]
\D	Match non numeric ： [^\d]
\s	Match all space characters , Equate to ： [\t\n\f\r\p{Z}]
\S	Match all non whitespace characters ： [^\s]
\f	Match a page break
\n	Match a line break
\r	Match a carriage return
\t	Match a tab
\v	Match a vertical tab
\p	matching CR/LF（ Equate to \r\n）, To match DOS Line terminator

4. Zero width assertion （ Pre check before and after ）

Both the first assertion and the last assertion belong to Non capture cluster （ Don't capture text , And it doesn't count against the combination meter ）.
Antecedent assertion is used to determine whether the matched format is prior to another determined format , The matching result does not contain the determined format （ As a constraint only ）.

for example , We want to get all the following $ The number after the symbol , We can use positive post assertion (?<=\$)[0-9\.]*.
This expression matches $ start , Followed by 0,1,2,3,4,5,6,7,8,9,. These characters can appear greater than or equal to 0 Time .

The zero width assertion is as follows ：

Symbol	describe
?=	Is asserting first - There is
?!	Negative antecedent assertion - exclude
?<=	Just after the assertion - There is
?<!	Negative post assertion - exclude

4.1 ?=... Is asserting first

?=... Is asserting first , The first part of the expression must be followed by ?=... Defined expression .

The returned result contains only the first part of the expression that meets the matching criteria .
Define a forward assertion to use (). Use a question mark and an equal sign inside the brackets ： (?=...).

What is asserted in advance is written after the equal sign in brackets .
for example , expression (T|t)he(?=\sfat) matching The and the, In parentheses, we define forward assertion (?=\sfat) , namely The and the Followed closely by ( Space )fat.

"(T|t)he(?=\sfat)" => The fat cat sat on the mat.

Practice online

4.2 ?!... Negative antecedent assertion

Negative antecedent assertion ?! Used to filter all matching results , The screening criteria are Not followed by the format defined in the assertion .
Is asserting first Definitions and Negative antecedent assertion equally , The difference is = Replace with ! That is to say (?!...).

expression (T|t)he(?!\sfat) matching The and the, And don't follow ( Space )fat.

"(T|t)he(?!\sfat)" => The fat cat sat on the mat.

Practice online

4.3 ?<= ... Just after the assertion

Just after the assertion Write it down as (?<=...) Used to filter all matching results , The screening criteria are It is preceded by the format defined in the assertion .
for example , expression (?<=(T|t)he\s)(fat|mat) matching fat and mat, And followed by The or the.

"(?<=(T|t)he\s)(fat|mat)" => The fat cat sat on the mat.

Practice online

4.4 ?<!... Negative post assertion

Negative post assertion Write it down as (?<!...) Used to filter all matching results , The screening criteria are It is not followed by the format defined in the assertion .
for example , expression (?<!(T|t)he\s)(cat) matching cat, And it doesn't follow The or the.

"(?<!(T|t)he\s)(cat)" => The cat sat on cat.

Practice online

5. sign

Flags are also called pattern modifiers , Because it can be used to modify the search results of expressions .
These signs can be used in any combination , It's also part of the whole regular expression .

sign	describe
i	Ignore case .
g	Global search .
m	Multiline modifier ： Anchor metacharacters ^$ The scope of work is at the beginning of each line .

5.1 Ignore case (Case Insensitive)

Modifier i Used to ignore case .
for example , expression /The/gi Means to search globally The, In the rear i Change its condition to ignore case , It becomes search the and The,g For global search .

"The" => The fat cat sat on the mat.

Practice online

"/The/gi" => The fat cat sat on the mat.

Practice online

5.2 Global search (Global search)

Modifier g Often used to perform a global search match , namely （ Not just the first matching , But back to all ）.
for example , expression /.(at)/g Represent search Any character （ Except for line breaks ）+ at, And return all the results .

"/.(at)/" => The fat cat sat on the mat.

Practice online

"/.(at)/g" => The fat cat sat on the mat.

Practice online

5.3 Multiline modifier (Multiline)

Multiline modifier m Often used to perform a multiline match .

As introduced before (^,$) Used to check whether the format is at the beginning or end of the string to be detected . But if we want it to work at the beginning and end of each line , We need to use multiline modifiers m.

for example , expression /at(.)?$/gm For lowercase characters a Followed by lowercase characters t , At the end, you can choose any character except line break . according to m Modifier , Now the expression matches the end of each line .

"/.at(.)?$/" => The fat
                cat sat
                on the mat.

Practice online

"/.at(.)?$/gm" => The fat
                  cat sat
                  on the mat.

Practice online

6. Greedy match and inert match (Greedy vs lazy matching)

By default, regular expressions use greedy matching pattern , In this mode it means matching as many substrings as possible . We can use ? Convert greedy matching pattern to lazy matching pattern .

"/(.*at)/" => The fat cat sat on the mat. 

Practice online

"/(.*?at)/" => The fat cat sat on the mat. 

Practice online

contribution

• Report a problem
• Open merge request
• Spread this document
• Contact me directly [email protected]

license

MIT Zeeshan Ahmad