Perl constructs not supported by this class:
The conditional constructs (?{
) and
(X
}?(
),condition
)X
|Y
The embedded code constructs (?{
) and
(code
}??{
),code
}
The embedded comment syntax (?#comment)
, and
The preprocessing operations \l
\u
, \L
, and \U
.
Constructs supported by this class but not by Perl:
Possessive quantifiers, which greedily match as much as they can and do not back off, even when doing so would allow the overall match to succeed.
Character-class union and intersection as described above.
Notable differences from Perl:
In Perl, \1
through \9
are always interpreted as back
references; a backslash-escaped number greater than 9
is treated as a back
reference if at least that many sub-expressions exist, otherwise it is interpreted, if
possible, as an octal escape. In this class octal escapes must always begin with a zero. In
this class, \1
through \9
are always interpreted as back
references, and a larger number is accepted as a back reference if at least that many
sub-expressions exist at that point in the regular expression, otherwise the parser will
drop digits until the number is smaller or equal to the existing number of groups or it is
one digit.
Perl uses the g
flag to request a match that resumes where the last match
left off. This functionality is provided implicitly by the Matcher
class: Repeated invocations of the find
method will resume where the last match left off, unless the
matcher is reset.
In Perl, embedded flags at the top level of an expression affect the whole expression. In this class, embedded flags always take effect at the point at which they appear, whether they are at the top level or within a group; in the latter case, flags are restored at the end of the group just as in Perl.
Perl is forgiving about malformed matching constructs, as in the expression
*a
, as well as dangling brackets, as in the expression abc]
, and
treats them as literals. This class also accepts dangling brackets but is strict about
dangling metacharacters like +, ? and *, and will throw a PatternSyntaxException
if it encounters them.
For a more precise description of the behavior of regular expression constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. F. Friedl, O'Reilly and Associates, 2002.