SRX and Java

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The SRX 2.0 standard is based on the ICU regular expression notation.

Many Java applications use Java's regular expressions to implement SRX because ICU4J (ICU for Java) does not provide support of ICU regular expressions.

As of version 1.7 Java has support for most of the Unicode-enabled features as described in ICU. For example in Java "\w" means "[\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Nd}]" like in ICU. Some ICU features can be replaced by an equivalent expression in Java, but some other features simply cannot be implemented in Java.

The following table shows the ICU and Java differences (assuming the UNICODE_CHARACTER_CLASS flag is set). The yellow entries denote a case where the ICU expression needs to be mapped to a Java equivalent (sometimes a complex one), and the red entries indicate the cases where the ICU expression cannot be mapped in Java.

Note: Starting in M28, the Okapi implementation of SRX no longer uses the ICU Regex option by default. Java patterns are used and Unicode processing enabled via the UNICODE_CHARACTER_CLASS flag. (You can test this for example in Ratel).


ICU Meta Character Java Equivalent ICU Description
\a same Match a BELL, \u0007
\A same Match at the beginning of the input. Differs from ^ in that \A will not match after a new line within the input.
\b, outside of a set same Match if the current position is a word boundary. Boundaries occur at the transitions between word (\w) and non-word (\W) characters, with combining marks ignored. And the option UREGEX_UWORD is assumed to be NOT set (default).
\b, within a set \b is invalid when within a set.
Use \u0008 instead.
Match a BACKSPACE, \u0008.
\B same Match if the current position is not a word boundary. And the option UREGEX_UWORD is assumed to be NOT set (default).
\cX same Match a control-X character.
\d same Match any character with the Unicode General Category of Nd (Number, Decimal Digit.)
\D same Match any character that is not a decimal digit.
\e same Match an ESCAPE, \u001B.
\E same Terminates a \Q ... \E quoted sequence.
\f same Match a FORM FEED, \u000C.
\G same Match if the current position is at the end of the previous match.
\n same Match a LINE FEED, \u000A.
\N{UNICODE CHARACTER NAME} Does not exists Match the named character.
\p{UNICODE PROPERTY NAME} same Match any character with the specified Unicode Property.
\P{UNICODE PROPERTY NAME} same Match any character not having the specified Unicode Property.
\Q same Quotes all following characters until \E.
\r same Match a CARRIAGE RETURN, \u000D.
\s same Match a white space character. White space is defined as [\t\n\f\r\p{Z}].
\S same Match a non-white space character.
\t same Match a HORIZONTAL TABULATION, \u0009.
\uhhhh same Match the character with the hex value hhhh.
\Uhhhhhhhh Does not exist Match the character with the hex value hhhhhhhh. Exactly eight hex digits must be provided, even though the largest Unicode code point is \U0010ffff.
\w same Match a word character. Word characters are [\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Nd}].
\W same Match a non-word character.
\x{hhhh} same Match the character with hex value hhhh
\xhh same Match the character with two digit hex value hh
\X Can approximate with complex regex (see extended grapheme support at bottom of page): Unicode Java Regex Equivalents Match a Grapheme Cluster.
\Z same Match if the current position is at the end of input, but before the final line terminator, if one exists.
\z same Match if the current position is at the end of input.
\0nnn same Match the character with octal value nnn.
\n same Back Reference. Match whatever the nth capturing group matched. n must be >1 and < total number of capture groups in the pattern.
[pattern] same Match any one character from the set. See UnicodeSet for a full description of what may appear in the pattern.
. same Match any character.
^ same Match at the beginning of a line.
$ same Match at the end of a line.
\ same Quotes the following character. Characters that must be quoted to be treated as literals are * ? + [ ( ) { } ^ $ | \ . /
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