Directory
Objective of this article
In 30 minutes you will understand what the regular expression is and have some basic knowledge of it so that you can use it in your own program or Web page.
How to use this tutorial
Most importantly-please give me 30 minutes , if you don't have the experience of using regular expressions, please don't try to get started in a second . Of course, if you're Superman, it's a different story.
Don't be intimidated by the complex expressions below, as long as you follow me step-by-step, you will find that the regular expression is not as difficult as you think. Of course, if you have finished this tutorial, it's normal to find out that you know a lot, but you can't remember nearly anything--I think that people who have not contacted the regular expression will have zero chance to remember the grammar mentioned above over 80% after reading this tutorial. Here is just to let you understand the basic principles, you need more practice, more use, to master regular expression.
In addition to being an introductory tutorial, this article attempts to become a regular expression grammar reference manual that you can use in your daily work. As far as the author's own experience is concerned, the goal is well done--you see, I haven't been able to write everything down myself, have I?
Text formatting conventions: Professional terms meta-character/grammar format part of a regular expression in a regular expression (for parsing) a description of a regular expression or part of a string that is searched for in it clears the formatting
What exactly is a regular expression?
When writing a program or Web page that handles strings, there is often a need to find strings that match some of the complex rules. Regular expressions are the tools used to describe these rules. In other words, regular expressions are code that records text rules.
It is likely that you have used the wildcard character (wildcard) for file lookup under Windows/dos, that is, * and?. If you want to find all the Word documents in a directory, you will search for *.doc. Here, * will be interpreted as any string. Like wildcard characters, regular expressions are also tools for text matching. It's just a more precise description of your needs than a wildcard--and, of course, the price is more complicated--for example, you can write a regular expression that looks for all of the 0 starts, followed by 2-3 digits, then a hyphen "-", The last is a 7-or 8-digit string (like 010-12345678 or 0376-7654321).
A regular expression is a tool for text matching, so this article mentions the search/lookup in a string many times, meaning that in a given string, look for the part that matches a given regular expression. It is possible that more than one part of a string satisfies a given regular expression, at which point each such part is called a match. The match may have three meanings in this article: one is an adjective, such as a string that matches an expression, a verb, such as a regular expression in a string, or a noun, which is just the "part of a string that satisfies a given regular expression".
Entry
The best way to learn regular expressions is to start with examples, to understand the examples, and to modify the examples themselves. A few simple examples are given here, and they are described in detail.
If you look for hi in an English novel, you can use the regular expression hi.
This is the simplest regular expression, which can exactly match such a string: consists of two characters, the first character is H, and the latter is I. Typically, the tool that handles regular expressions provides an option to ignore the case, and if this option is selected, it can match any of the four Hi,hi,hi,hi.
Unfortunately, many words contain the two consecutive characters of Hi, such as Him,history,high and so on. If you look it up with Hi, the side of Hi will also be found. If you want to find the word "hi" accurately, we should use \bhi\b.
\b is a special code that is prescribed by regular expressions (well, some people call it metacharacters, metacharacter), representing the beginning or end of a word, the boundary of a word. Although English words are usually separated by spaces or punctuation or wrapping, \b does not match any of these word delimiters, it matches only one position . (If a more precise statement is needed, \b matches a position where the first and last characters are not all (one is, one is not or does not exist) \w)
If you're looking for hi, not far behind. Follow a Lucy, you should use \bhi\b.*\blucy\b.
Here,. is another meta character that matches any character except the newline character. * is also a meta character, but it represents not a character, nor a position, but a quantity--it specifies that the contents of the front can be repeated repeatedly to make the entire expression match. So,. * Connecting together means any number of characters that do not contain a newline. Now the meaning of \bhi\b.*\blucy\b is obvious: first a word hi, then any arbitrary character (but not a newline), and finally the word Lucy.
If you use some other meta characters at the same time, we can construct a more powerful regular expression. For example, the following example:
0\d\d-\d\d\d\d\d\d\d\d matches a string that starts with 0, then two digits, then a hyphen "-", and finally 8 digits (that is, China's phone number). Of course, this example can only match a case with an area code of 3 digits.
The \d here is a new meta character that matches any number (0, or 1, or 2, or ...). -is not a meta character, only matches itself--the hyphen.
To avoid so many annoying repetitions, we can also write this expression: 0\d{2}-\d{8}. Here \d {2} ({8}) means that the preceding \d must be repeated 2 times (8 times) consecutively.
Testing Regular Expressions
If you don't think regular expressions are hard to read or write, you're either a genius or you're not from Earth. The syntax of regular expressions is a headache, even for people who often use it. Because it is difficult to read and write and error prone, it is necessary to create a tool to test regular expressions.
Because some of the details of regular expressions are not the same in different environments, this tutorial describes the behavior of the regular expressions under Microsoft. Net 2.0, so I'd like to introduce you to one. NET under the tool regex Tester. First you make sure that the. Net Framework 2.0 is installed, and then the Regex Tester is downloaded. This is a green software, download and then open the compression package, directly run RegexTester.exe on it.
The following is a screenshot of the Regex Tester Runtime:
Metacharacters
Now you know a few useful meta characters, such as \b,., *, and \d. Of course, there are more meta characters available, such as \s matching any whitespace, including spaces, tabs (tab), line breaks, Chinese full-width spaces, and so on. \w matches letters or numbers or underscores or kanji.
Let's try more examples here:
\ba\w*\b matches words that begin with the letter A--first at the beginning of a word (\b), then there is the letter A, then any number of letters or numbers (\w*), and finally the end of the word (\b) (OK, now let's talk about what the word in the regular expression means: several successive \w. Yes, it does not really matter to the tens of thousands of things with the same name that are to be memorized when learning English.
\d+ matches 1 or more consecutive digits. Here the + is and * similar to the meta character, the difference is the * match repeat any time (may be 0 times), and + will match repeat 1 or more times.
\b\w{6}\b matches a word that is just 6 letters/digits.
| code | Description |
|---|---|
| . | |
| \w | matches letters or numbers or underscores or kanji |
| \s | match any whitespace |
| match number | |
| \b | |
| ^ | start of matching string |
| $ | end of match string |
The meta character ^ (and the number 6 symbol on the same keys) and $ and \b are somewhat similar and match one position. ^ matches the beginning of the string you want to use to find, and the $ matches the end. These two code is very useful when validating the input content, for example a website if asks you to fill in the QQ number must be 5 digits to 12 digits, may use: ^\d{5,12}$.
The {5,12} here is similar to {2} described earlier, except that {2} matches only a little more than a few 2 times, {5,12} is the number of repetitions can not be less than 5 times, not more than 12 times, otherwise all do not match.
Because of the use of ^ and $, the entire string entered should be used to match the \d{5,12}, which means that the entire input must be 5 to 12 digits, so if the input QQ number can match the regular expression, then it will meet the requirements.
Similar to the option of ignoring case, some regular expression processing tools also have an option to handle multiple rows. If this option is selected, the meaning of ^ and $ becomes the beginning and end of the matching line.
Word Escape
If you want to look up the meta characters themselves, such as if you're looking for, or *, there's a problem: You can't specify them because they'll be interpreted as other meanings. Then you have to use \ To remove the special meaning of these characters. Therefore, you should use \. and \*. Of course, to find \ itself, you have to use \.
For example: www\.unibetter\.com matching www.unibetter.com,c:\\windows matching c:\Windows.
Repeat
You've seen the previous *,+,{2},{5,12} these several matching repetitions of the way. The following are all qualifiers in the regular expression (a specified number of codes, such as *,{5,12}):
| Code/Syntax | Description |
|---|---|
| * | Repeat 0 or more times |
| + | Repeat one or more times |
| ? | Repeat 0 times or once |
| N | Repeat n times |
| {N,} | Repeat N or more times |
| {N,m} | Repeat N to M times |
Here are some examples of using duplicates:
windows\d+ match Windows followed by 1 or more digits
13\D{9} match 13 followed by 9 digits (China Mobile number)
^\w+ matches the first word of a line (or the first word of the entire string, exactly which meaning depends on the option setting)
Character class
To find numbers, letters, or numbers, whitespace is simple because there are metacharacters that correspond to these characters, but what if you want to match character sets (such as vowel a,e,i,o,u) without predefined meta characters?
Simply, you just need to list them in brackets, like [aeiou] to match any one English vowel, [.?!] Match punctuation (. or? or!) (English sentences usually end with only three punctuation).
We can also easily specify a range of characters, as [0-9] the meaning of the representative is exactly the same as \d: a number, the same [a-z0-9a-z_] is exactly equivalent to \w (if only in English).
The following is a more complex expression: \ (? 0\d{2}[)-]?\d{8}.
This expression can match phone numbers in several formats, such as (010) 88886666, or 022-22334455, or 02912345678. Let's do some analysis of it: First, an escape character \ (it can occur 0 or 1 times). And then a 0, followed by 2 digits (\d{2}), or--or one of the spaces, which appears 1 times or does not appear (?).), and finally 8 digits (\d{8}). Unfortunately, it can also match 010 12345678 or (022-87654321) for "incorrect" formats. To resolve this issue, find the answer below in this tutorial.
Anti-righteousness
Sometimes you need to find characters that are not part of a simple definition of a character class. For example, if you want to find any other character that is in addition to a number, then you need to use the following antisense:
| Code/Syntax | Description |
|---|---|
| \w | Match any characters that are not letters, numbers, underscores, Chinese characters |
| \s | Match any character that is not a blank symbol |
| \d | Match any number of non-numeric characters |
| \b | Matches a position that is not the beginning or end of a word |
| [^x] | Matches any character other than X |
| [^aeiou] | Match any character other than aeiou these letters |
Example: \s+ matches a string that does not contain whitespace characters.
<a[^>]+> matches a string enclosed in angle brackets that starts with a.
Replace
OK, now it's time to fix the 3-bit or 4-bit area code problem. The substitution of regular expressions means that there are several rules, and if any one of them is to be matched, the specific method is to separate the different rules. Don't you understand? Okay, look at the example:
0\d{2}-\d{8}|0\d{3}-\d{7} This expression can match two phone numbers separated by a hyphen: one is a three-bit area code, a 8-bit local number (such as 010-12345678), a 4-digit area code, and a 7-bit local number (0376-2233445).
\ (0\d{2}\) [-]?\d{8}|0\d{2}[-]?\d{8} This expression matches the phone number of the 3-bit area code, where the area code can be enclosed in parentheses or not, and can be separated by a hyphen or space between the area code and the local number, or without spacing. You can try a replacement | To extend this expression to also support 4-bit area code.
\d{5}-\d{4}|\d{5} This expression is used to match the postal code of the United States. The rules for the U.S. ZIP Code are 5 digits, or 9 digits with a hyphen interval. The reason to give this example is because it illustrates a problem: the order is important when using substitution . If you change it to \d{5}|\d{5}-\d{4, it will only match the 5-bit ZIP code (and the first 5 digits of the 9-bit zip code). The reason is that each branching condition is tested from left to right when a match is made, and if a branch is satisfied, the other replacement condition is not regulated.
windows98| windows2000| Windosxp This example is to tell you that substitution can be used not only for two rules, but also for a variety of rules.
Group
We've mentioned how to repeat a single character (just add a qualifier to the character), but what if you want to repeat more than one character? You can specify a subexpression (also called a grouping) with parentheses, and then you can specify the number of repetitions of the subexpression, and you can do other things with the sub-expression (described later).
(\d{1,3}\.) {3}\d{1,3} is a simple IP address matching expression. To understand this expression, analyze it in the following order: \d{1,3} matches 1 to 3 digits, (\d{1,3}\.) {3} matches a three-digit number plus an English period (this whole is this group) repeats 3 times, and finally adds a number of one to three digits (\d{1,3}).
Unfortunately, it will also match 256.300.888.999 this impossible IP address (each number in an IP address cannot be greater than 255). Off the counter, like the 24-hour third-quarter writers do not know this, Khan ...). If you can use arithmetic comparisons, you may be able to solve this problem simply, but the regular expression does not provide any function of mathematics, so you can use a lengthy grouping, selection, character class to describe a correct IP address: (2[0-4]\d|25[0-5]|[ 01]?\d\d?) \.) {3} (2[0-4]\d|25[0-5]| [01]?\d\d?].
The key to understanding this expression is understanding 2[0-4]\d|25[0-5]| [01]?\d\d, I will not elaborate here, you should be able to analyze the meaning of it.
Back reference
After you use parentheses to specify a subexpression, the text that matches the subexpression (that is, what this group captures) can be further processed in an expression or other program. By default, each grouping automatically has a group number, which is left to right, is marked with the left parenthesis of the group, the first group that appears is 1, the second is 2, and so on.
A back reference is used to repeat the search for a previously grouped text. For example, \1 represents grouping 1 matching text. Hard to understand? Take a look at the example:
\b (\w+) \b\s+\1\b can be used to match repeated words, like go, Kitty Kitty. The first is a word, which is more than one letter or number (\b (\w+) \b) between the beginning and the end of the word, followed by 1 or several blank characters (\s+), and finally the word (\1) that matches the previous one.
You can also specify the group name of the subexpression yourself. To specify a group name for a subexpression, use this syntax: (? <word>\w+) (or change the angle bracket to ' also OK: (? '). Word ' \w+ '), so that the \w+ group name is specified as Word. To reverse reference to the content captured by this grouping, you can use \k<word>, so the previous example can be written like this: \b (? <word>\w+) \b\s+\k<word>\b.
When using parentheses, there are many specific uses of the syntax. Some of the most commonly used are listed below:
| capture | |
|---|---|
| (exp) | matches exp and captures the text into the automatically named group |
| (? <name>exp) | matches exp and captures the text in a group named name, or it can be written as (?) Name ' exp ' |
| matching Exp, do not capture matching text or assign group number to this grouping | |
| 0 wide assertion | |
| (? =exp) | |
| (? <=exp) | matches the location of the exp |
| (?! EXP) | |
| (? <!exp) | |
| note | |
| (? #comment) | this type of group does not have any effect on the processing of regular expressions , used to provide comments for people to read the |
We have discussed the first two syntaxes. The third (?: EXP) does not change the way regular expressions are handled, except that the matching content of such groups is not captured in a group like the first two.
0 Wide Assertion
The next four are used to find things before or after certain content, but not the content, that is, they are used as \b,^,$ to specify a position that should satisfy certain conditions (assertions), so they are also called 0-wide assertions. It's best to take an example to illustrate:
(? =exp) is also called the 0-width positive lookahead assertion, which asserts that the position in which it appears is followed by the expression exp. For example \b\w+ (? =ing\b), matches the front part of the word with ing ending (except for the part of ING), such as finding I ' m singing while you ' re dancing. When it matches sing and Danc.
(? <=exp) also known as the 0 width is reviewed later to assert that it asserts that the position itself appears to precede the expression exp. For example (? <=\bre) \w+\b matches the second half of a word that begins with re (except for parts other than re), for example, when looking for reading a book, it matches ading.
If you want to add a comma to every three digits in a very long number (plus, of course, from the right), you can look for parts that need commas in front and inside: (? <=\d) \d{3}) *\b, the result is 234567890 when you use it to find 1234567890.
The following example uses both assertions: (? <=\s) \d+ (? =\s) matches numbers that are separated by whitespace (again emphasizing, excluding these whitespace characters).
Negative 0 Wide Assertion
We mentioned earlier how to find a method that is not a character or a character that is not in a certain character class (antisense). But what if we just want to make sure that a character doesn't appear, but doesn't want to match it ? For example, if we want to find a word that has the letter Q in it, but Q is not followed by the letter u, we can try this:
\b\w*q[^u]\w*\b matches words that contain the letter Q that is not followed by the letter U . But if you do more testing (or if you're sensitive enough to see it directly), you'll find that if Q appears at the end of the word, like Iraq,Benq, the expression will go wrong. This is because [^u] always matches one character, so if Q is the last character of the word, then [^u] will match the word delimiter after Q (possibly a space, or a period or something), and the \w*\b will match the next word, so \b\w*q[^u]\w*\ B will be able to match the entire Iraq fighting. A negative 0-wide assertion solves such a problem because it matches only one location and does not consume any characters. Now, we can solve this problem like this: \b\w*q. u) \w*\b.
0 width Negative lookahead assertion (?!) EXP), asserting that the expression exp is not matched at the back of this position. For example: \d{3} (?! \d) matches three digits, and this three-digit number cannot be followed by a number; \b (?! ABC) +\b matches words that do not contain the \w string ABC.
In the same way, we can use (? <!exp), 0 width is reviewed after the assertion to assert that the front of this position can not match the expression Exp: (? <![ A-z]) \d{7} matches a seven-digit number that is not preceded by a lowercase letter.
A more complex example: (?<=< (\w+) >). * (?=<\/\1>) matches the contents of a simple HTML tag that does not contain attributes. (a)? (\w+) >) specifies a prefix such as a word that is enclosed in angle brackets (for example, a possible <b>) followed by a. * (any string), and finally a suffix (?=<\/\1>). Notice the \/in the suffix, which uses the previously mentioned word escape \1 is a reverse reference, which refers to the first set of captures, the preceding (\w+) matching content, so that if the prefix is actually <b>, the suffix is </b>. The entire expression matches the content between <b> and </b> (reminders again, excluding prefixes and suffixes themselves).
Comments
Another use of parentheses is to include a comment through a syntax (? #comment). For example: 2[0-4]\d (? #200 -249) |25[0-5] (? #250-255) | [01]?\d\d? (? #0-199).
To include annotations, it is best to enable the "ignore whitespace in mode" option so that you can add any space, Tab, and line breaks when you write an expression, which will be ignored when you actually use it. When this option is enabled, all text that ends in # after the end of the line will be ignored as comments.
For example, we can write one of the preceding expressions:
(? <= # asserts that the prefix of the text to be matched
< (\w+) > # Find the letter or number (that is, the html/xml tag) enclosed by the angle bracket
) # prefix ends
. * # Match any text
(? = # assertion to match the suffix of the text
<\/\1> # Find the bracketed content: preceded by a "/", followed by a previously captured label
) # suffix End
Greed and laziness
When a regular expression contains a qualifier that accepts duplicates, the usual behavior is to match as many characters as possible (subject to the entire expression being matched). Consider this expression: A.*b, which will match the longest string with a beginning of a, ending with B. If you use it to search for Aabab, it will match the entire string aabab. This is called greedy match.
Sometimes we need a lazy match, which is to match as few characters as possible . The qualifier given above can be converted to lazy match mode, just add a question mark after it. this. * means matching any number of repetitions, but using the least repetition if the whole match succeeds. Now look at the lazy version of the example:
A.*?b matches the shortest string, starting with a and ending with B. If applied to Aabab, it would match AaB and AB (why is the first match aab rather than AB?) Simply put, because regular expressions have another rule, higher precedence than lazy/greedy rules: The first match has the highest priority--the match that begins earliest Wins).
| *? | Repeat any time, but try to 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 |
Processing options
These options can be used to change the way a regular expression is handled, such as ignoring case, handling multiple rows, and so on. Here's the. Common regular Expression options in net:
| name | description |
|---|---|
| ignorecase (Ignore case) | matches are case-insensitive. |
| multiline (multiline mode) | change ^ and $ so that they match at the beginning and end of any line, not just at the beginning and ending of the entire string. (In this mode, the exact meaning of $ is to match the previous position and the position before the end of the string.) |
| singleline (single-line mode) | change ., so that it matches every character (including the newline character \ n). |
| ignorepatternwhitespace (ignore whitespace) | ignores non-escaped whitespace in an expression and enables annotations by # tag. |
| righttoleft (lookup from right to left) | match from right to left instead of left to right. |
| captures only groups that have been explicitly named. | |
One question that is often asked is: Is it possible to use only one of multiple-line and single-line patterns at the same time? The answer is: No. There is no relationship between the two options, except that their names are more similar (and so confusing).
Balance Group/recursive matching
Note: The balancing group syntax described here is supported by the. Net Framework; Other languages/libraries do not necessarily support this feature, or support this feature but require different syntax.
Sometimes we need to match a nested hierarchical structure such as (100 * (50 + 15)), simply using \ (. +\) will only match the content between the leftmost left parenthesis and the rightmost closing parenthesis (we're talking about greedy mode, and lazy mode has the following problem). If the original string has an unequal number of opening and closing parentheses, such as (5/(3 + 2)), the number of the two matches will not be equal. Is there a way to match the content between the longest and the paired brackets in such a string?
To avoid (and/or confuse) your brain, we'll use angle brackets instead of parentheses. Now our question becomes how to capture the contents of the longest pairing in a string such as xx <aa <bbb> <bbb> aa> yy?
Here you need to use the following syntax constructs:
If you are not a programmer (or you are a programmer who is not familiar with the concept of stacks), you understand the above three kinds of grammar: the first one is to write a "group" on the blackboard, the second is to erase a "group" from the blackboard, and the third is to see if there are any "group" written on the blackboard, If there is, continue to match the Yes section, otherwise it will match the no part.
What we need to do is to write a "group" on the blackboard every touch of an opening parenthesis, rub one on each of the closing brackets, and then see if there is any on the blackboard-if there is more left parenthesis than the right parenthesis, the match should fail.
< #最外层的左括号
[^<>]* #最外层的左括号后面的不是括号的内容 ((
?) Open ' < ' #碰到了左括号, write an "open"
[^<>]* #匹配左括号后面的不是括号的内容) on the blackboard
+
(
? -open ' >) #碰到了右括号, erase an "Open"
[^<>]* #匹配右括号后面不是括号的内容
) +
) *
( Open) (?!)) #在遇到最外层的右括号前面, judge if there is any "Open" on the blackboard, and if so, the match fails
> #最外层的右括号One of the most common applications of the balancing group is to match HTML, and the following example matches the nested <div> Tags: <div[^>]*>[^<>]* ((?) Open ' <div[^>]*> ' [^<>]*] + (? -open ' </div>) [^<>]*] +) * ( Open) (?!)) </div>.
There's something else that's not mentioned.
I've described a lot of elements of constructing regular expressions, and there's something I haven't mentioned. The following is a list of elements that are not mentioned, including syntax and a simple description. You can find more detailed references on the web to learn about them-when you need them. If you install the MSDN Library, you can also find detailed documentation of. NET regular expressions in it.
| \a | Alarm character (the effect of printing it is a computer beep) |
| \b | It's usually a word demarcation, but if you use a backspace in a character class |
| \ t | tab, tab |
| \ r | Enter |
| \v | Vertical tab |
| \f | Page breaks |
| \ n | Line feed |
| \e | Escape |
| \0nn | Characters with octal code as nn in ASCII code |
| \xnn | Characters with hexadecimal code NN in ASCII code |
| \unnnn | Characters with hexadecimal code nnnn in Unicode code |
| \cn | ASCII control characters. Like \CC, the representative of CTRL + C |
| \a | String at the beginning (similar to ^, but not affected by multiline options) |
| \z | End of string, or end of line (not affected by multiple-line options) |
| \z | End of string (similar to $ but not affected by multiline options) |
| \g | The beginning of the current search |
| \p{name} | A character class named name in Unicode, such as \p{isgreek} |
| (? >exp) | Greedy subexpression |
| (? <x>-<y>exp) | Balance Group |
| (? im-nsx:exp) | Changing processing options in subexpression exp |
| (? im-nsx) | Change the processing options for the part that follows the expression |
| (? (exp) yes|no) | Take exp as a 0 wide forward assertion, and if you can match at this position, use Yes as an expression for this group; |
| (? (exp) Yes) | Ditto, just use an empty expression as no |
| (? (name) yes|no) | If the group named name captures the content, use Yes as an expression; |
| (? (name) Yes) | Ditto, just use an empty expression as no |
Contact author
Well, I admit, I lied to you, you must have spent more than 30 minutes reading here. Believe me, it's my fault, not because you're too stupid. The reason why I say "30 minutes" is to give you confidence and patience to continue. Now that you've seen it, that proves my plot was a success. It's good to have a taste like this.
To complain about me, or feel that I can actually do better, or have any other questions, welcome to my blog for discussion.
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