Many ListProc command lines take regular expressions as arguments. A
regular expression is a group of symbols which describe a unique string
of characters. An example of a simple regular expression is the word "donkey".
In a group of words, the regular expression "donkey" matches
only other instances of the word "donkey" and nothing else. So
if we had a text file and did a search for "donkey" every time
that word appeared in the text it would show up in our search. This definition
can be expanded if we define the period "." as a wild card replacement
for a single character. Then the regular expression ".onkey"
would include "honkey" or "tonkey" or "bonkey"
as well as "donkey". We can continue to expand on the definition
by adding the asterisk to the period " .* " as a substitute for
any number of characters. Then the regular expression "don.* "
will not only include "donkey" but will include all strings of
characters beginning with "don" including "don" itself.
A search of a text file for "don.* " will turn up dongle, donkey,
donner, dondoodit, dondiddle, etc. Regular expression matching, therefore,
puts together a whole set of rules that allow you to test whether a string
fits into a specific syntactic shape. You can also search a string for
a substring that fits a pattern, and just as importantly, you can replace
one string with another. The command line of a ListProc command is more
like a regular expression as used in a database search.
Regular expressions have a syntax in which a few characters are special
constructs and the rest are "ordinary". An ordinary character
is a simple regular expression which matches that character and nothing
else. The special characters are `$', `^', `.', `*', `+', `?', `[', `]'
and `\'. Any other character appearing in a regular expression is ordinary,
unless a `\' precedes it.
For example, `f' is not a special character, so it is ordinary, and
therefore `f' is a regular expression that matches the string `f' and no
other string. (It does not match the string `ff'.) Likewise, `o'
is a regular expression that matches only `o'.
Any two regular expressions A and B can be concatenated. The result
is a regular expression which matches a string if A matches some amount
of the beginning of that string and B matches the rest of the string.
As a simple example, we can concatenate the regular expressions `f'
and `o' to get the regular expression `fo', which matches only the string
`fo'. Still trivial.
The following are the characters and character sequences which have
special meaning within regular expressions. Any character not mentioned
here is not special; it stands for exactly itself for the purposes of searching
and matching.
`.'
is a special character that matches anything except a newline. Using
concatenation, we can make regular expressions like `a.b' which matches
any three-character string which begins with `a' and ends with `b'.
`*'
is not a construct by itself; it is a suffix, which means the preceding
regular expression is to be repeated as many times as possible. In `fo*',
the `*' applies to the `o', so `fo*' matches `f' followed by any number
of `o''s. The case of zero `o''s is allowed: `fo*' does match `f'.
`*' always applies to the *smallest* possible preceding expression.
Thus, `fo*' has a repeating `o', not a repeating `fo'. The matcher processes
a `*' construct by matching, immediately, as many repetitions as can be
found. Then it continues with the rest of the pattern. If that fails, backtracking
occurs, discarding some of the matches of the `*''d construct in case that
makes it possible to match the rest of the pattern. For example, matching
`c[ad]*ar' against the string `caddaar', the `[ad]*' first matches `addaa',
but this does not allow the next `a' in the pattern to match. So the last
of the matches of `[ad]' is undone and the following `a' is tried again.
Now it succeeds.
`+'
`+' is like `*' except that at least one match for the preceding pattern
is required for `+'. Thus, `c[ad]+r' does not match `cr' but does match
anything else that `c[ad]*r' would match.
`?'
`?' is like `*' except that it allows either zero or one match for the
preceding pattern. Thus, `c[ad]?r' matches `cr' or `car' or `cdr', and
nothing else.
`[ ... ]'
`[' begins a "character set", which is terminated by a `]'.
In the simplest case, the characters between the two form the set. Thus,
`[ad]' matches either `a' or `d', and `[ad]*' matches any string of `a''s
and `d''s (including the empty string), from which it follows that `c[ad]*r'
matches `car', etc.
Character ranges can also be included in a character set, by writing
two characters with a `-' between them. Thus, `[a-z]' matches any lower-case
letter. Ranges may be intermixed freely with individual characters, as
in `[a-z$%.]', which matches any lower case letter or `$', `%' or period.
Note that the usual special characters are not special any more inside a character set. A completely different set of special characters exists inside character sets: `]', `-' and `^'. To include a `]' in a character set, you must make it the first character. For example, `[]a]' matches `]' or `a'. To include
a `-', you must use it in a context where it cannot possibly indicate
a range: that is, as the first character, or immediately after a range.
`[^ ... ]'
`[^' begins a "complement character set", which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]' matches all characters
except letters and digits. Note that the ^ has to be within brackets.
Outside of brackets it has a different meaning as mentioned below. `^'
is not special in a character set unless it is the first character. The
character following the `^' is treated as if it were first (it may be a
`-' or a `]').
`^'
is a special character that matches the empty string -- but only if
at the beginning of a line in the text being matched. Otherwise it fails
to match anything. Thus, `^foo' matches a `foo' which occurs at the beginning
of a line.
`$'
is similar to `^' but matches only at the end of a line. Thus, `xx*$'
matches a string of one or more `x''s at the end of a line.
`\'
has two functions: it quotes the above special characters (including
`\'), and it introduces additional special constructs. If you wanted to
search for the dollar sign within a text you would have to use \$ in place
of just $ since the dollar sign has a special meaning. Because `\' quotes
special characters, `\$' is a regular expression which matches only `$',
and `\[' is a regular expression which matches only `[', and so on.
For the most part, `\' followed by any character matches only that character.
However, there are several exceptions: characters which, when preceded
by `\', are special constructs. Such characters are always ordinary when
encountered on their own.
No new special characters will ever be defined. All extensions to the
regular expression syntax are made by defining new two-character constructs
that begin with `\'.
`\|'
specifies an alternative. Two regular expressions A and B with `\|'
in between form an expression that matches anything that either A or B
will match. Thus, `foo\|bar' matches either `foo' or `bar' but no other
string. `\|' applies to the largest possible surrounding expressions. Only
a surrounding `\( ... \)' grouping can limit the grouping power of `\|'.
Full backtracking capability exists when multiple `\|''s are used.
`\( ... \)'
is a grouping construct that serves three purposes:
1. To enclose a set of `\|' alternatives for other operations. Thus,
`\(foo\|bar\)x' matches either `foox' or `barx'.
2. To enclose a complicated expression for the postfix `*' to operate
on. Thus, `ba\(na\)*' matches `bananana', etc., with any (zero or more)
number of `na''s.
3. To mark a matched substring for future reference. This last application
is not a consequence of the idea of a parenthetical grouping; it is a separate
feature which happens to be assigned as a second meaning to the same `\(
... \)' construct because there is no conflict in practice between the
two meanings. Here is an explanation of this feature:
`\DIGIT'
After the end of a `\( ... \)' construct, the matcher remembers the
beginning and end of the text matched by that construct. Then, later on
in the regular expression, you can use `\' followed by DIGIT to mean "match
the same text matched the DIGIT'th time by the `\( ... \)' construct."
The `\( ... \)' constructs are numbered in order of commencement in the
regexp.
The strings matching the first nine `\( ... \)' constructs appearing
in a regular expression are assigned numbers 1 through 9 in order of their
beginnings. `\1' through `\9' may be used to refer to the text matched
by the corresponding `\( ... \)' construct.
For example, `\(.*\)\1' matches any string that is composed of two identical
halves. The `\(.*\)' matches the first half, which may be anything, but
the `\1' that follows must match the same exact text.
`\b'
matches the empty string, but only if it is at the beginning or end
of a word. Thus, `\bfoo\b' matches any occurrence of `foo' as a separate
word. `\bball\(s\|\)\b' matches `ball' or `balls' as a separate word.
`\B'
matches the empty string, provided it is *not* at the beginning or end
of a word.
`\<'
matches the empty string, but only if it is at the beginning of a word.
`\>'
matches the empty string, but only if it is at the end of a word.
`\w'
matches any word-constituent character.
`\W'
matches any character that is not a word-constituent. What the `\( ...
\)' groupings matched.
Here are examples of commands that use regular expressions:
lists global 'health|mental'~death
The above will compile a list of lists that contain either the word
'health' or 'mental' in either their list name or description comment but
will exclude lists with the word 'death'. The way you should read 'health|mental'~death
out loud is; "health or mental but not death".
lists global move$&dan$
will search for all lists containing BOTH the characters 'move' AND
'dan' so that move$ will return both movement and movies and dan$ will
return both dancing and danger. But in order for you to receive a reply,
the list will have to contain BOTH words. So a list about Dangerous Movies
will show up in your search as well as a list about Movement and Dancing.
ignore <listname> <password> bart@^ar..+beta.org
This example will ignore a user named bart whether he posts from ar1.beta.org
or from ar2.beta.org or art.beta.org.
alias <listname> <password> (.+)@host.domain \1@domain
will take anything inside the () and store it in \1 which is then accessed. For example, if homer@cs.domain sends a message, it will be translated to homer@domain.
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