Huazhong University of Science and Technology JAVA Programming Language Lecture 8:String, Regular Expression

Transcription

1 JAVA Programming Language Lecture 8:String, Regular Expression Chengwei Zhang ( 张成伟 ) School of Electronic Information and Communications Huazhong University of Science and Technology Mar. 2016

2 Outline String Basic I/O Comments Java Programming Language 2

3 What is a String? A string is a series of characters treated as a single unit A string may include letters, digits and various special characters, such as +, -, *, /, $ and others A string is an object/instance of class String abc, 1234utj.., hometown, Java Programming Language 3

4 String basic information Location: java.lang.string Definition: public final class String extends Object implements Serializable, CharSequence, Comparable<String> Note: The content of a String object is immutable A rich set of constructors and operations proposed to use Java Programming Language 4

5 String constructors String * All private fields + String() + String(byte[] bytes) + String(byte[] bytes, String charsetname) + String(char[] value) + String(String original) + String(StringBuffer buffer) + Use different ways to initialize a String object Provide 15 constructors for initializing String objects in a variety of ways Java Programming Language 5

6 Useful operations in String String + charat(int index):char + compareto(string anotherstring): int + comparetoignorecase(string str): int + concat(string str): String + contains(charsequence s): boolean + format( ): String + getbytes( ): byte[] + indexof( ): int + isempty(): boolean + length(): int + split( ): String[] + substring( ): String + trim(): String + valueof( ): String Java Programming Language 6

7 Group operations of String methods Normal Access Length(), charat(), getchars() Compare Strings equals(), equalsignorecase(), compareto(), regionmathes() Locating chars and substring in Strings indexof(), lastindexof(), substring() Concatenating Strings Concat(), + Type translate to Strings valueof() Miscellaneous methods replace(), tolowercase(), touppercase(), trim(), tochararray(), Java Programming Language 7

8 Samples of String Create String objects using direct assignment Create String objects using constructors Use String methods to operate the string object Java Programming Language 8

9 Comments Character StringBuffer StringBuilder Dynamic String objects Store chars based on the capacity Capacity expands dynamically to handle additional data Java Programming Language 9

10 PART II: REGULAR EXPRESSION Java Programming Language 10

11 What is Regular Expression? (Regular Expression) A way to describe a set of strings based on common characteristics shared by each string in the set (Java Tutorial) Use to search, edit, or manipulate text and data (Regular Expression) Sequences of characters and symbols that define a set of strings. Useful for validating input and ensuring a particular format of the data A kind of general feature for variant tools Languages: Perl, Python, TCL, PHP, C# Tools: grep, awk Java Programming Language 11

12 Regular Expression Package Package: java.util.regex Main classes in java.util.regex Pattern A compiled representation of a regular expression An immutable regular expression Matcher A regular expression matching operation The engine that interprets the pattern and performs match operations against an input string PatternSyntaxException Syntax error in a regular expression pattern(unchecked exception) Java Programming Language 12

13 Basic working flow of RE An example Use the Pattern and Matcher to locate or find strings Pattern: define the string format rule Can use the String object or regular expression to define Matcher: store matched results v.s. String.contains( ) true or false Metacharacters < ( [ { \ ^ - = $! ] } )? * +. > Example: some modification of the example Java Programming Language 13

14 MetaCharacter Table Construct Description. Any character (may or may not match line terminators) \d A digit: [0-9]; matches all the digits \D A non-digit: [^0-9]; matches non-digits \s A whitespace character: [ \t \n \x0b \f \r]; matches all the whitespaces \S A non-whitespace character: [^\s]; matches non-spaces \w A word character: [a-za-z_0-9]; matches word characters \W A non-word character: [^\w]; matches non-word characters MetaCharacter Sample: \d, \D, \s, \D, \w, \W in the regular expression? Note: Inclusive the start index, exclusive the end index Java Programming Language 14

15 Quantifiers Table Greedy Reluctant Possessive Meaning X? X?? X?+ X, once or not at all X* X*? X*+ X, zero or more times X+ X+? X++ X, one or more times X{n} X{n}? X{n}+ X, exactly n times X{n,} X{n,}? X{n,}+ X, at least n times X{n,m} X{n,m}? X{n,m}+ X, at least n but not more than m times Quantifiers allow to specify the number of occurrences to match Different samples to see what happen? (Group capture support?) Regex: a?, a*, a+; a{3}; a{3,6}; String:, a, aaaaa, ababaaab ; aa, aaa, aaaa ; aaaaaaaaa ; Group captures: (dog){3} v.s. dog{3}; [abc]{3} v.s. abc{3} Difference among Greedy, Reluctant, and Possessive? Java Programming Language 15

16 Differences: Greedy, Reluctant, and Possessive? Greedy From the backward direction Reluctant From the forward direction Possessive From the forward direction Samples: Regex:.*foo,.*?foo,.*+foo String: xfooxxxxxxfoo Java Programming Language 16

17 Other Issues Capturing Groups Phone numbr: 1[0-9]{10} Boundary Matchers Backreferences Java Programming Language 17

18 Tokenizer Partition String into individual substrings Use delimiter Java offers java.util.stringtokenizer Regular expression Sequence of characters and symbols Define set of strings Class Pattern An immutable regular expression Class Match A regular expression matching operation Java Programming Language 18

19 Regular expression Character Matches Character Matchers \d Any digit \D Any non-digit \w Any word character \W Any non-word character \s Any whitespace \S Any non-whitespace Quantifier Matches * Matches zero or more occurrences of the pattern. + Matches one or more occurrences of the pattern.? Matches zero or one occurrences of the pattern. {n} Matches exactly n occurrences. {n, } Matches at least n occurrences. {n, m} Matches between n and m (inclusive) occurrences. Java Programming Language 19

20 Thanks