Regular Expressions-first entry threshold

Source: Internet
Author: User
Tags alphabetic character regular expression book

Concepts and basic elements: 1. metacharacters: § Regular expressions consist of some ordinary characters and some meta-characters (meta characters).      Ordinary characters include uppercase and lowercase letters and numbers, while metacharacters have special meanings. A. Common meta-characters:
Metacharacters Describe
\ Marks the next character as a special character, or a literal character, or a backward reference, or an octal escape. For example, ' n ' matches the character "n". ' \ n ' matches a line break. The sequence ' \ \ ' matches "\" and "\ (" Matches "(".
. Matches any single character except "\ n". To match any character including ' \ n ', use a pattern like ' [. \ n] '. For example, the regular expression r.t matches these strings: Rat, Rut, R T, but does not match root.
^ Matches the starting position of the input string. For example, the regular expression ^when in is able to match the start of the string "When in the course of human events", but does not match "what is in the".
$ Matches the end position of the input string. For example, the regular expression book$ is able to match the end of the string "This was a book", but cannot match the string "They has many books".
[xyz] The character set is combined. Matches any one of the characters contained. For example, ' [ABC] ' can match ' a ' in ' plan '.
[A-z] The character range. Matches any character within the specified range. For example, ' [A-z] ' can match any lowercase alphabetic character in the ' a ' to ' Z ' range. You can also make multiple intervals, such as regular expressions [a-za-z] that match any uppercase and lowercase letters.
\b Matches a word boundary, which is the position between a word and a space. For example, ' er\b ' can match ' er ' in ' never ', but not ' er ' in ' very '.
\d Matches a numeric character. equivalent to [0-9].
\s Matches any whitespace character, including spaces, tabs, page breaks, and so on.
\w Matches any word character that includes an underscore. Equivalent to ' [a-za-z0-9_] '.
\ n Matches a line break. Equivalent to \x0a.
\ r Matches a carriage return character. Equivalent to \x0d.
B. Repetition:
Metacharacters Describe
* Matches the preceding subexpression 0 or more times. For example, zo* can match "z" and "Zoo". * Equivalent to {0,}.
+ Matches the preceding subexpression one or more times. For example, ' zo+ ' can match "Zo" and "Zoo", but not "Z". + equivalent to {1,}.
? Matches the preceding subexpression 0 or one time. For example, "Do (es)?" can match "do" in "do" or "does".? Equivalent to {0,1}.
{n} N is a non-negative integer. Matches the determined n times. For example, ' o{2} ' cannot match ' o ' in ' Bob ', but can match two o in ' food '.
{n n is a non-negative integer. Match at least n times. For example, ' o{2,} ' cannot match ' o ' in ' Bob ', but can match all o in ' Foooood '. ' O{1,} ' is equivalent to ' o+ '. ' O{0,} ' is equivalent to ' o* '.
{n,m} Both M and n are non-negative integers, where n <= m. Matches at least n times and matches up to m times. For example, "o{1,3}" will match the first three o in "Fooooood". ' o{0,1} ' is equivalent to ' O? '. Note that there can be no spaces between a comma and two numbers.
C. Counter-Justification:
Metacharacters Describe
\b Matches a non-word boundary. ' er\b ' can match ' er ' in ' verb ', but cannot match ' er ' in ' Never '.
\d Matches a non-numeric character. equivalent to [^0-9].
\s Matches any non-whitespace character. equivalent to [^ \f\n\r\t\v].
\w Matches any non-word character. Equivalent to ' [^a-za-z0-9_] '.
[^XYZ] Negative character set. Matches any character that is not contained. For example, ' [^ABC] ' can match ' P ' in ' plain '.
[^a-z] A negative character range. Matches any character that is not in the specified range. For example, ' [^a-z] ' can match any character that is not within the range of ' a ' to ' Z '.
D. Other: 1.x|y match condition X or condition y, two matching criteria are logical "or" (or) operation. 2. We have already mentioned how to repeat a single character (directly following the character with a qualifier). But what if you want to repeat multiple characters? You can use parentheses to specify sub-expressions (also called groupings) such as (\d{1,3}\.)   {3}\d{1,3} is a simple IP address matching expression, but it will also match the 256.300.888.999 IP address that cannot exist. 3. Lazy Qualifiers

Metacharacters

Describe

*?

Repeat any number of times, but repeat as little as possible

+?

Repeat 1 or more times, but repeat as little as possible

??

Repeat 0 or 1 times, but repeat as little as possible

{n,m}?

Repeat N to M times, but repeat as little as possible

{N,}?

Repeat more than n times, but repeat as little as possible

Simple Regular Expression application:
    1. Registered account number: 5-20 letters, numbers or underscores, the first character must be a letter
      §:^[A-ZA-Z]\W{4,19};
    2. Phone number with area code: XXX-XXXXXXXX or Xxxx-xxxxxxx
      §: (^[0]\d{2}-\d{8}) | (^[0]\d{3}-\d{7});
    3. The correct IP address description: (0-255). (0-255). (0-255). (0-255)
      §: ((2[0-4]\d|25[0-5]|[ 01]?\d\d?) \.) {3} (2[0-4]\d|25[0-5]| [01]?\d\d?];

Regular Expressions-first entry threshold

Contact Us

The content source of this page is from Internet, which doesn't represent Alibaba Cloud's opinion; products and services mentioned on that page don't have any relationship with Alibaba Cloud. If the content of the page makes you feel confusing, please write us an email, we will handle the problem within 5 days after receiving your email.

If you find any instances of plagiarism from the community, please send an email to: info-contact@alibabacloud.com and provide relevant evidence. A staff member will contact you within 5 working days.

A Free Trial That Lets You Build Big!

Start building with 50+ products and up to 12 months usage for Elastic Compute Service

  • Sales Support

    1 on 1 presale consultation

  • After-Sales Support

    24/7 Technical Support 6 Free Tickets per Quarter Faster Response

  • Alibaba Cloud offers highly flexible support services tailored to meet your exact needs.