what does the regular expression $$ evaluate to?
A:
That can depend on the exact type of REs in use. In the commonly-used POSIX basic REs, $ means, basically, "end of line". So this would match a line followed by an empty line.
unwind
2009-04-29 08:07:11
@unwind: "this would match a line followed by an empty line" -- It wouldn't. $ is zero-width. ;-)
Tomalak
2009-04-29 08:12:27
Okay ... I tested it, but maybe not in the correct environment (I used EditPad Pro in Windows, since I already had it running).
unwind
2009-04-29 08:20:43
In EPP's search box, matches are always in Multiline mode, so $ matches the end of a line or the end of the document. But it doesn't match any characters.
Alan Moore
2009-04-29 11:59:03
+2
A:
It just depends on the underlying regex engine. In vim it would match a dollar sign at the end of line, for example. I'd guess that Posix would require the engine to match the same as '$' since that literal has 0 length as Tomalak pointed out.
soulmerge
2009-04-29 08:18:03
+1
A:
It depends on the underlying regex engine. But in the .Net framework System.Text.RegularExpressions.Regex class, $ within regular expressions specifies that the match must occur at the end of the string, before \n at the end of the string, or at the end of the line, so $$ will match everything!
($$ has another meaning in replacement patterns, which substitutes a single "$" literal.)
M. Jahedbozorgan
2009-04-29 08:32:57
"$$" will match everything, in terms of "every character in the string"? Are you sure?
Tomalak
2009-04-29 09:09:28
"$$" will match everything, in terms of "every string".//The following will return true for any given textprivate static bool IsMatch(string text){ return new System.Text.RegularExpressions.Regex("$$").IsMatch(text);}
M. Jahedbozorgan
2009-04-29 09:28:55
Hm... When you look at it that way, it's true. You made it sound like it was a special property of "$$", but it is equally true for "^" or "$" or ".*". The key question remains unanswered - what does it *match*?
Tomalak
2009-04-29 09:40:02
Yes, "^" or "$" or ".*" also match everything! because they are actually equal in functionality to "" (zero-width string) which matches everything.
M. Jahedbozorgan
2009-04-29 10:04:32
A:
Normally is a matcher for end-of-line but this depend on which language are you using. Without this specific information is difficult to answer.
BTW, in csh means the process number. It is not a regular expression.
Luis
Luixv
2009-04-29 08:36:55
+2
A:
That construct is unlikely to occur except in languages that allow variable interpolation in the regex. In that case it most likely will match the process id of the current process (since $$ is generally a variable that holds the process id).
Chas. Owens
2009-04-29 14:29:33
+3
A:
in Perl, embedding $$ into a regular expression will result in the PID of the perl process being inserted
# Suppose perl has a PID of 5432
my $str1 = "some";
my $str2 = "5432";
print "1 match: $str1" if $str1 =~ /^$$/;
print "2 match: $str2" if $str2 =~ /^$$/;
Output:
2 match: 5432
Inserting a single $ will match the end of line.
dsm
2009-04-29 14:46:46
+2
A:
It depends on the programming language and regexp engine.
- In Perl,
/$$/matches the PID (process id) of the current process (see more in dsm's answer). - In Ruby, both
/$$/and/$/match the empty string before the first newline, or the end of the string if there are no newlines in the string. - In Python, both
re.search('$$', s)andre.search('$', s)match the empty string before the last newline, or the end of the string if there are no newlines in the string. - (Other languages or regexp engines may behave differently.)
Please note that the flags (usually the s and the m flags) associated with the regexp affect what $ matches. The items above use the default flags.
pts
2009-04-30 23:12:14