SRX and Java

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 * ? + [ ( ) { } ^ $ | \ . /