1. Regular Expressions
A regular expression define a search pattern for strings. This pattern may match one or several times or not at all for a given string. The abbreviation for regular expression is "regex".
A simple example for a regular expression is a (literal) string. For example the regex "Hello World" will match exactly the phrase "Hello World". Another example for a regular expression is "." (dot) which matches any single character; it would match for example "a" or "z" or "1".
Regular expressions can be used to search, edit and manipulate text.
Regular expressions are used in several programming languages, e.g. Java but also Perl, Groovy, etc. Unfortunately each language / program supports regex slightly different.
The pattern defined by the regular expression is applied on the string from left to right. Once a source character has been used in a match, it cannot be reused. For example the regex "aba" will match "ababababa" only two times (aba_aba__).
Some of the following examples use JUnit to validate the result. You should be able to adjust them in case if you don't want to use JUnit. To learn about JUnit please see JUnit Tutorial .
The following is an overview of regular expressions. This chapter is supposed to be a references for the different regex elements.
Table 1.
| Regular Expression | Description |
|---|---|
| . | Matches any sign |
| ^regex | regex must match at the beginning of the line |
| regex$ | Finds regex must match at the end of the line |
| [abc] | Set definition, can match the letter a or b or c |
| [abc][vz] | Set definition, can match a or b or c followed by either v or z |
| [^abc] | When a "^" appears as the first character inside [] when it negates the pattern. This can match any character except a or b or c |
| [a-d1-7] | Ranges, letter between a and d and figures from 1 to 7, will not match d1 |
| X|Z | Finds X or Z |
| XZ | Finds X directly followed by Z |
| $ | Checks if a line end follows |
The following metacharacters have a pre-defined meaning and make certain common pattern easier to use, e.g. \d instead of [0..9].
Table 2.
| Regular Expression | Description |
|---|---|
| \d | Any digit, short for [0-9] |
| \D | A non-digit, short for [^0-9] |
| \s | A whitespace character, short for [ \t\n\x0b\r\f] |
| \S | A non-whitespace character, for short for [^\s] |
| \w | A word character, short for [a-zA-Z_0-9] |
| \W | A non-word character [^\w] |
| \S+ | Several non-whitespace characters |
A quantifier defines how often an element can occur. The symbols ?, *, + and {} define the quantity of the regular expressions
Table 3.
| Regular Expression | Description | Examples |
|---|---|---|
| * | Occurs zero or more times, is short for {0,} | X* - Finds no or several letter X, .* - any character sequence |
| + | Occurs one or more times, is short for {1,} | X+ - Finds one or several letter X |
| ? | Occurs no or one times, ? is short for {0,1} | X? -Finds no or exactly one letter X |
| {X} | Occurs X number of times, {} describes the order of the preceding liberal | \d{3} - Three digits, .{10} - any character sequence of length 10 |
| {X,Y} | .Occurs between X and Y times, | \d{1,4}- \d must occur at least once and at a maximum of four |
| *? | ? after a qualifier makes it a "reluctant quantifier", it tries to find the smallest match. |
原文地址：http://www.vogella.de/articles/JavaRegularExpressions/article.html
