For many configuration entries which are used for searching of text patterns the
system may use PERL compatible regular expressions known usually as Unix style
regular expressions.
Explanation of regular expression's syntax
Any single character matches itself, unless it is a meta-character with a
special meaning listed in the following table.
A series of characters in the expression matches that series of characters
in the target string. So the pattern "hello" entered as regular expression
would match ``hello'' in the target string. Quite simple.
Summary table of used metacharacters
Character | Meaning
|
---|
^ | start of line
|
$ | end of line
|
. | any character
|
\ | quote next character
|
* | match zero or more, similar to {0,}
|
+ | match one or more, similar to {1,}
|
? | match zero or one, similar to {0,1}
|
{n} | Match exactly n times
|
{n,} | Match at least n times
|
{n,m} | Match at least n but not more than m times
|
[aeiou0-9] | match a, e, i, o, u, and 0 thru 9 ;
|
[^aeiou0-9] | match anything but a, e, i, o, u, and 0 thru 9
|
\w | matches an alphanumeric character (including "_")
|
\W | a nonalphanumeric
|
\d | matches a numeric character
|
\D | a non-numeric
|
\s | matches any space (same as [ \t\n\r\f])
|
\S | a non space
|
\1..\9 | backreferences
|
You can force characters that normally function as metacharacters to be
interpreted literally by prefixing them with a ``\''. For example, "^" match
beginning of string, but "\^" match character "^", "\\" match "\" and so on.
Characters may be specified using a metacharacter syntax much like that used
in C: ``\n'' matches a newline, ``\t'' a tab, ``\r'' a carriage return,
``\f'' a form feed, etc. More generally, \xnn, where nn is a string of
hexadecimal digits, matches the character whose ASCII value is nn. If you
need wide (UniCode) character code, you can use '\x{nnnn}', where 'nnnn' -
one or more hexadedimal digits.
You can specify a character class, by enclosing a list of characters in [],
which will match any one character from the list. If the first character
after the ``['' is ``^'', the class matches any character not in the list.
Within a list, the ``-'' character is used to specify a range, so that a-z
represents all characters between ``a'' and ``z'', inclusive. If you want
``-'' itself to be a member of a class, put it at the start or end of the
list, or escape it with a backslash.
The following all specify the same class of three characters: [-az], [az-],
and [a\-z]. All are different from [a-z], which specifies a class containing
twenty-six characters. If you want ']' you may place it at the start of list
or escape it with a backslash.
Examples of ranges: [\n-\x0D] match any of #10,#11,#12,#13. [\d-t]
match any digit, '-' or 't'. []-a] match any char from ']'..'a'.
Finally, the ``.'' metacharacter matches any character except ``\n'' (unless
you use /s modifier).
You can specify a series of alternatives for a pattern using ``|'' to
separate them, so that fee|fie|foe will match any of ``fee'', ``fie'', or
``foe'' in the target string (as would f(e|i|o)e). The first alternative
includes everything from the last pattern delimiter (``('', ``['', or the
beginning of the pattern) up to the first ``|'', and the last alternative
contains everything from the last ``|'' to the next pattern delimiter. For
this reason, it's common practice to include alternatives in parentheses, to
minimize confusion about where they start and end.
Alternatives are evaluated from left to right, so the first alternative found
for which the entire expression matches, is the one that is chosen. This
means that alternatives are not necessarily greedy. For example: when matching
foo|foot against ``barefoot'', only the ``foo'' part will match, as that is
the first alternative tried, and it successfully matches the target string.
(This might not seem important, but it is important when you are capturing
matched text using parentheses.)
Also remember that ``|'' is interpreted as a literal within square brackets,
so if you write [fee|fie|foe] you're really only matching [feio|].
The bracketing construct ( ... ) may also be used for define r.e.
subexpressions. Subexpressions are numbered based on the left to right order
of their opening parenthesis. First subexpression has number '1'.
You may use \w, \d and \s within character classes.
\1 through \9 are interpreted as backreferences. \ matches previously
matched subexpression #. For example: '(.)\1+' match 'aaaa' and 'cc'.
'(.+)\1+' also match 'abab' and '123123'.
By default, the ^ character means the beginning of the string, the $ character
the end (or before the newline at the end) and perl does certain optimizations
with the assumption that the string contains only one line.
Embedded newlines will not be matched by ``^'' or ``$''.