The source for the standard Python library module regex_syntax.py
contains documentation on regex.set_syntax. It can do modest things
such as swap the meaning of \(...\) with (...). However beware that
its effect is global and changing the regex syntax permanently in your
program may break some of the standard library modules (e.g. rfc822).
A good policy is to bracket a set of regex.compile() calls with calls
that set and restore the desired regex syntax. (A better policy is
usually to use the standard syntax :-)
> Has anyone written a perl regular expression module for python?
I don't think so. In any case, what exactly should such a module do?
Surely it can't change Python's syntax into Perl...
However, I can try and translate your fragment of Perl code into
approximately similar Python. I'll assume there's a variable "line"
containing the current line. (Actually, I guessed about the meanin
gof Perl's "grep" function. Someone correct me if I'm wrong.)
# Put this in the global part of your file (outside any functions)
import string, regex, sys
subjprog = regex.compile('^Subject:\(.*\)', regex.casefold)
# This is assumedly executed in a loop over the lines of a file, e.g.
for line in sys.stdin.readlines():
if subjprog.match(line) >= 0:
subject = subjprog.group(1) # Extract subexpression 1
subject = string.strip(subject) # Strip leading & trailing whitespace
subscribing = regex.match('^[ \t]subscribe', subject, regex.casefold)
unsubscribing = regex.match('^[ \t]unsubscribe', subject, regex.casefold)
...
Note that I mix precompiled regular expressions and "in-line" regular
expressions here -- the .group() method is only available for compiled
regular expressions, but for the other two using in-line calls is more
compact.
> If there is a WWW server with some decent documentation on the regular
> expression syntax used in Python?
I don't know, but here's the Emacs-19 info about regular expressions.
Python regexps support all the listed \ sequences except \s.
Syntax of Regular Expressions
=============================
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 `\'; no new special characters will be defined. 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. To do something nontrivial, you need to
use one of the special characters. Here is a list of them.
`. (Period)'
is a special character that matches any single character 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
one `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 `*'-modified construct in
case that makes it possible to match the rest of the pattern.
For example, matching `ca*ar' against the string `caaar', the
`a*' first tries to match all three `a's; but the rest of the
pattern is `ar' and there is only `r' left to match, so this try
fails. The next alternative is for `a*' to match only two `a's.
With this choice, the rest of the regexp matches successfully.
`+'
Is a suffix character similar to `*' except that it requires that
the preceding expression be matched at least once. So, for
example, `ca+r' will match the strings `car' and `caaaar' but not
the string `cr', whereas `ca*r' would match all three strings.
`?'
Is a suffix character similar to `*' except that it can match the
preceding expression either once or not at all. For example,
`ca?r' will match `car' or `cr'; 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 one `a' or one `d', and `[ad]*'
matches any string composed of just `a's and `d's (including the
empty string), from which it follows that `c[ad]*r' matches `cr',
`car', `cdr', `caddaar', 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 `$', `%' or period.
Note that the usual special characters are not special any more
inside a character set. A completely different set of special
characters exists inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `[]a]' matches `]' or `a'. To include a
`-', write `---', which is a range containing only `-'. To
include `^', make it other than the first character in the set.
`[^ ... ]'
`[^' begins a "complement character set", which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]'
matches all characters except letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).
Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.
`^'
is a special character that matches the empty string, but only if
at the beginning of a line in the text being matched. Otherwise
it fails to match anything. Thus, `^foo' matches a `foo' which
occurs at the beginning of a line.
`$'
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' matches a string of one `x' or more at the end of a line.
`\'
has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.
Because `\' quotes special characters, `\$' is a regular
expression which matches only `$', and `\[' is a regular
expression which matches only `[', and so on.
Note: for historical compatibility, special characters are treated
as ordinary ones if they are in contexts where their special meanings
make no sense. For example, `*foo' treats `*' as ordinary since there
is no preceding expression on which the `*' can act. It is poor
practice to depend on this behavior; better to quote the special
character anyway, regardless of where is appears.
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. Here is a table of `\'
constructs.
`\|'
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 to handle multiple uses of
`\|'.
`\( ... \)'
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' strings.
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 strings matching the first nine `\( ... \)' constructs
appearing in a regular expression are assigned numbers 1 through
9 in order that the open-parentheses appear in the regular
expression. `\1' through `\9' may be used to refer to the text
matched by the corresponding `\( ... \)' construct.
For example, `\(.*\)\1' matches any newline-free 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.
`\`'
matches the empty string, provided it is at the beginning of the
buffer.
`\''
matches the empty string, provided it is at the end of the buffer.
`\b'
matches the empty string, provided it is at the beginning or end
of a word. Thus, `\bfoo\b' matches any occurrence of `foo' as a
separate word. `\bballs?\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, provided it is at the beginning of a
word.
`\>'
matches the empty string, provided it is at the end of a word.
`\w'
matches any word-constituent character. The editor syntax table
determines which characters these are.
`\W'
matches any character that is not a word-constituent.
`\sCODE'
matches any character whose syntax is CODE. CODE is a character
which represents a syntax code: thus, `w' for word constituent,
`-' for whitespace, `(' for open-parenthesis, etc. *Note
Syntax::.
`\SCODE'
matches any character whose syntax is not CODE.
Here is a complicated regexp, used by Emacs to recognize the end of
a sentence together with any whitespace that follows. It is given in
Lisp syntax to enable you to distinguish the spaces from the tab
characters. In Lisp syntax, the string constant begins and ends with
a double-quote. `\"' stands for a double-quote as part of the regexp,
`\\' for a backslash as part of the regexp, `\t' for a tab and `\n'
for a newline.
"[.?!][]\"')]*\\($\\|\t\\| \\)[ \t\n]*"
This contains four parts in succession: a character set matching
period, `?' or `!'; a character set matching close-brackets, quotes or
parentheses, repeated any number of times; an alternative in
backslash-parentheses that matches end-of-line, a tab or two spaces;
and a character set matching whitespace characters, repeated any
number of times.
--Guido van Rossum, CWI, Amsterdam <Guido.van.Rossum@cwi.nl>
<URL:http://www.cwi.nl/cwi/people/Guido.van.Rossum.html>