package gnu.regexp;
Brief Background
A regular expression consists
of a character string where some characters are given special meaning with regard
to pattern matching. Regular expressions have been in use from the early days of
computing, and provide a powerful and efficient way to parse, interpret and search
and replace text within an application.
Supported Syntax
Within a
regular expression, the following characters have special meaning:
^
matches at the beginning of a line1$
matches at the end of a line2\A
matches the start of the entire string\Z
matches the end of the entire string
.
matches any single character3\d
matches any decimal digit\D
matches any non-digit\n
matches a newline character\r
matches a return character\s
matches any whitespace character\S
matches any non-whitespace character\t
matches a horizontal tab character\w
matches any word (alphanumeric) character\W
matches any non-word (alphanumeric) character\x
matches the character x, if x is not one of the above listed escape sequences.
Within a character class expression, the following sequences have special meaning if the syntax bit RE_CHAR_CLASSES is on:[abc]
matches any character in the set a, b or c[^abc]
matches any character not in the set a, b or c[a-z]
matches any character in the range a to z, inclusive
A leading or trailing dash will be interpreted literally.
[:alnum:]
Any alphanumeric character[:alpha:]
Any alphabetical character[:blank:]
A space or horizontal tab[:cntrl:]
A control character[:digit:]
A decimal digit[:graph:]
A non-space, non-control character[:lower:]
A lowercase letter[:print:]
Same as graph, but also space and tab[:punct:]
A punctuation character[:space:]
Any whitespace character, including newline and return[:upper:]
An uppercase letter[:xdigit:]
A valid hexadecimal digit
(abc)
matches whatever the expression abc would match, and saves it as a subexpression. Also used for grouping.(?:...)
pure grouping operator, does not save contents(?#...)
embedded comment, ignored by engine\n
where 0 < n < 10, matches the same thing the nth subexpression matched.
a|b
matches whatever the expression a would match, or whatever the expression b would match.
?
matches the preceding expression or the null string*
matches the null string or any number of repetitions of the preceding expression+
matches one or more repetitions of the preceding expression{m}
matches exactly m repetitions of the one-character expression{m,n}
matches between m and n repetitions of the preceding expression, inclusive{m,}
matches m or more repetitions of the preceding expression
?
, the repeating operator will stop at the smallest number of
repetitions that can complete the rest of the match.Unsupported Syntax
Some flavors of regular expression utilities support
additional escape sequences, and this is not meant to be an exhaustive list. In
the future, gnu.regexp
may support some or all of the following:
(?=...)
positive lookahead operator (Perl5)(?!...)
negative lookahead operator (Perl5)(?mods)
inlined compilation/execution modifiers (Perl5)\G
end of previous match (Perl5)\b
word break positional anchor (Perl5)\B
non-word break positional anchor (Perl5)\<
start of word positional anchor (egrep)\>
end of word positional anchor (egrep)[.symbol.]
collating symbol in class expression (POSIX)[=class=]
equivalence class in class expression (POSIX)
Java Integration
In a Java environment, a regular expression operates on a string of Unicode characters,
represented either as an instance of java.lang.String
or as an array
of the primitive char
type. This means that the unit of matching is
a Unicode character, not a single byte. Generally this will not present problems
in a Java program, because Java takes pains to ensure that all textual data uses
the Unicode standard.
Because Java string processing takes care of certain escape
sequences, they are not implemented in gnu.regexp
. You should be aware
that the following escape sequences are handled by the Java compiler if found in
the Java source:
In addition, note that the\b
backspace\f
form feed\n
newline\r
carriage return\t
horizontal tab\"
double quote\'
single quote\\
backslash\xxx
character, in octal (000-377)\uxxxx
Unicode character, in hexadecimal (0000-FFFF)
\u
escape sequences are meaningful anywhere in a Java program,
not merely within a singly- or doubly-quoted character string, and are converted
prior to any of the other escape sequences. For example, the line gnu.regexp.RE
exp = new gnu.regexp.RE("\u005cn");
\u005c
with a backslash, then converting \n
to a newline.
By the time the RE constructor is called, it will be passed a String object containing
only the Unicode newline character. The POSIX character classes (above), and
the equivalent shorthand escapes (\d
, \w
and the like)
are implemented to use the java.lang.Character
static functions whenever
possible. For example, \w
and [:alnum:]
(the latter only
from within a class expression) will invoke the Java function Character.isLetterOrDigit()
when executing. It is always better to use the POSIX expressions than a range
such as [a-zA-Z0-9]
, because the latter will not match any letter characters
in non-ISO 9660 encodings (for example, the umlaut character, "ü
").
Reference Material
perlre(1)
man page (Perl Programmer's Reference Guide)regcomp(3)
man page
(GNU C)gawk(1)
man page (GNU utilities)sed(1)
man page (GNU utilities)ed(1)
man page (GNU utilities)grep(1)
man page (GNU utilities)regexp(n)
and regsub(n)
man
pages (TCL)Notes
1 but see the REG_NOTBOL and REG_MULTILINE
flags
2 but see the REG_NOTEOL and REG_MULTILINE
flags
3 but see the REG_MULTILINE flag