Friday, 11 May 2012

Regular Expression

We need to use regular expression frequently in text processing for search, parse, validation or XML document integrity. Java provide us a package called java.util.regex to make life easier for regular expression. Bellow I have summarized the things as I need to use Java Regex so frequently :)

Common matching symbols:

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


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


Characters Description
x The character x
\\ The backslash character
n The character with octal value 0n (0<=n<=7)
nn The character with octal value 0nn (0<=n<=7)
mnn The character with octal value 0mnn (0<=m<=3, 0<=n<=7)
\xhh The character with hexadecimal value 0xhh
\uhhhh The character with hexadecimal value 0xhhhh
\t The tab character ('\u0009')
\n The newline (line feed) character ('\u000A')
\r The carriage-return character ('\u000D')
\f The form-feed character ('\u000C')
\a The alert (bell) character ('\u0007')
\e The escape character ('\u001B')
\cx The control character corresponding to x


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.
A simple example for case insensitive URL matching using java Regex given bellow:

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