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This article will explain in detail what regular expressions are in JAVA. The editor thinks it is very practical, so I share it for you as a reference. I hope you can get something after reading this article.

] js regular expression basic syntax (essence):

Many languages, including Perl, PHP, Python, JavaScript, and JScript, support regular expressions for text processing, and some text editors implement advanced search-and-replace functions with regular expressions. So the JAVA language is no exception. Regular expression has gone beyond the limitations of a certain language or system and has become a widely used tool. We can use it to solve some practical problems encountered in actual development.

Regular expressions are not included in JDK1.3 and previous JDK versions. If you want to use regular expressions in Java, you must use a regular expression library provided by a third party. The most famous is that the Jakarta-ORO,Jakarta-ORO library was formerly called OROMatcher, which is an open source package given by Daniel Savarese to Jakarta Project. The first thing to use is to create an instance variable that implements the PatternCompiler interface to create a "schema compiler". The class that implements this interface in Jakarta-ORO is Perl5Compiler, which is fully compatible with Perl5's regular expressions. Jakarta-ORO is very easy to use, very efficient, and the supported regular expression syntax is very complete. The only drawback is that it is not a standard package in JDK. Support for regular expressions API is provided from JDK1.4, which is located in the java.util.regex package. Since there is already a standard API, this book will use java.util.regex for regular expression related operations.

1. Basic knowledge of regular expressions

1.1 period symbol

Suppose you are playing English Scrabble and want to find three-letter words that start with the letter "t" and end with the letter "n". In addition, suppose there is an English dictionary, and you can search all its contents with regular expressions. To construct this regular expression, you can use a wildcard, the period symbol. Thus, the complete expression is "t.n", which matches "tan", "ten", "tin" and "ton", as well as "tween", "tpn" or even "t n", and many other meaningless combinations. This is because the period symbol matches all characters, including spaces, Tab characters, and even newline characters:

1.2 square bracket symbol

To solve the problem that the matching range of period symbols is too wide, you can specify characters that look meaningful in square brackets ("[]"). At this point, only the characters specified in square brackets participate in the match. That is, the regular expression "t [aeio] n" only matches "tan", "Ten", "tin" and "ton". But "Toon" does not match, because you can only match the order in square brackets

Characters:

1.3 "or" symbol

If you want to match "toon" in addition to all the words above, you can use the "|" operator. The basic meaning of the "|" operator is the "OR" operation. To match "toon", use the regular expression "t (a | e | I | o | oo) n". Square expansions cannot be used here because square brackets only allow matching of individual characters; parentheses "()" must be used here. Parentheses can also be used to group.

1.4 symbol indicating the number of matches

The following table shows the syntax of regular expressions:

Table 1.1 regular expression syntax

Suppose we want to search for American Social Security numbers in a text file. The format of this number is 999-99-9999. The regular expression used to match it is shown in figure 1. In regular expressions, a hyphen ("-") has a special meaning, representing a range, such as 0 to 9. Therefore, when matching a hyphen in a social security number, it is preceded by an escape character "/".

Suppose you want hyphens to appear or not to appear when searching-that is, 999-99-9999 and 999999999 are in the correct format. At this point, you can add "?" after the hyphen. Quantity limit symbol.

One format of an American license plate is four numbers plus two letters. Its regular expression is preceded by the numeric part "[0-9] {4}" and the alphabetic part "[Amurz] {2}".

1.5 "No" symbol

The "^" symbol is called the "no" symbol. If used in square brackets, "^" indicates a character that you do not want to match. For example, the regular expression in figure 4 matches all words, except those that begin with the letter "X".

1.6 parentheses and white space symbols

The "/ s" symbol is a white space symbol that matches all white space characters, including Tab characters. If the strings match correctly, how do you extract the month part next? Just add a parenthesis around the month to create a group, and then use ORO API to extract its value.

1.7 other symbols

For simplicity, you can use some shortcut symbols created for common regular expressions. As shown below:

/ t: tab, equivalent to / u0009

/ n: newline character, equivalent to / u000A

/ d: represents a number, equivalent to [0-9]

/ D: stands for non-numeric, equivalent to [^ 0-9]

/ s: represents white space characters such as newline characters, Tab tabs, etc.

/ S: represents a non-white space character

/ w: alphabetic characters, equivalent to [a-zA-Z_0-9]

/ W: non-alphabetic characters, equivalent to [^ / w]

For example, in the previous example of a social security number, we can use "/ d" wherever "[0-9]" appears.

The following is the procedure I have sorted out: for reference:

Package org.luosijin.test; import java.util.regex.Matcher; import java.util.regex.Pattern; / * regular expression * @ version V. * @ author Luo Sijin * @ date-* / public class Regex {/ * * @ param args * @ author Luo Sijin * @ date-afternoon: * / public static void main (String [] args) {Pattern pattern = Pattern.compile ("baggage") Matcher matcher = pattern.matcher ("bbg"); System.out.println (matcher.matches ()); System.out.println (pattern.matches ("baggage", "bbg")); / / verify the zip code System.out.println (pattern.matches ("{}", "); System.out.println (" / / d {} ",")) / / verify the phone number System.out.println (pattern.matches ("{,} / /-+", ")); getDate (" Nov, "); charReplace (); / / verify the ID card: determine whether a string is an ID number, that is, whether it is an or digit. System.out.println (pattern.matches ("^ / / d {} | / / d {} $", ")); getString (" D:/dir/test.txt "); getChinese (" welcome to china, Jiangxi Fengxin, welcome, you! "); validateEmail (" luosijin@.com ") } / * date extraction: extract month to * @ param str * @ author Luo Sijin * @ date-afternoon: * / public static void getDate (String str) {String regEx= "([a-zA-Z] +) | / / s + {,}, / / s * {}"; Pattern pattern = Pattern.compile (regEx); Matcher matcher = pattern.matcher (str) If (! matcher.find ()) {System.out.println ("date format error!"); return;} System.out.println (matcher.group ()); / / the index value of the packet is from the beginning, so the method to take the first grouping is m.group () instead of m.group (). } / * character substitution: in this example, all places in a string that contain one or more consecutive "a" are replaced with "A". * * @ author Luo Sijin * @ date-AM: * / public static void charReplace () {String regex = "a +"; Pattern pattern = Pattern.compile (regex); Matcher matcher = pattern.matcher ("okaaaa LetmeAseeaaa aa booa"); String s = matcher.replaceAll ("A"); System.out.println (s) } / * string extraction * @ param str * @ author Luo Sijin * @ date-AM: * / public static void getString (String str) {String regex = ". + / (. +) $"; Pattern pattern = Pattern.compile (regex); Matcher matcher = pattern.matcher (str); if (! matcher.find ()) {System.out.println ("incorrect file path format!") ; return;} System.out.println (matcher.group ());} / * * Chinese extraction * @ param str * @ author Luo Sijin * @ date-in the morning: * / public static void getChinese (String str) {String regex = "[/ / uE-//uFFF] +"; / [/ / uE-//uFFF] is the Chinese character Pattern pattern = Pattern.compile (regex); Matcher matcher = pattern.matcher (str) StringBuffer sb = new StringBuffer (); while (matcher.find ()) {sb.append (matcher.group ());} System.out.println (sb);} / * * verify Email * @ param email * @ author Luo Sijin * @ date-AM: * / public static void validateEmail (String email) {String regex = "[- a-zA-Z] + @ [- a-zA-Z] + /. [- a-zA-Z] +" Pattern pattern = Pattern.compile (regex); Matcher matcher = pattern.matcher (email); if (matcher.matches ()) {System.out.println ("this is legal Email");} else {System.out.println ("this is illegal Email") } this is the end of this article on "what are regular expressions in JAVA?". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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