08-13-2012
Quote:
Originally Posted by
alister
That's incorrect. In a POSIX ERE, ^ is only an anchor when it is the first character in an expression.
\|^ should be portable, but, sadly, some implementations botch ^ handling. Some (perhaps all?) mawk implementations definitely do not handle this correctly.
Regards,
Alister
No. What you say is true for a BRE; not for an ERE. Quoting from IEEE Std. 1003.1-2008, P190, L6174-6183:
Quote:
9.4.9 ERE Expression Anchoring
An ERE can be limited to matching strings that begin or end a line; this
is called ‘‘anchoring''. The <circumflex> and <dollar-sign> special
characters shall be considered ERE anchors when used anywhere outside
a bracket expression. This shall have the following effects:
1. A <circumflex> ('ˆ') outside a bracket expression shall anchor the
expression or subexpression it begins to the beginning of a string; such
an expression or subexpression can match only a sequence starting at
the first character of a string. For example, the EREs "ˆab" and "(ˆab)"
match "ab" in the string "abcdef", but fail to match in the string
"cdefab", and the ERE "aˆb" is valid, but can never match because the
'a' prevents the expression "ˆb" from matching starting at the first
character.
2. ... ... ...
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LEARN ABOUT DEBIAN
bup-margin
bup-margin(1) General Commands Manual bup-margin(1)
NAME
bup-margin - figure out your deduplication safety margin
SYNOPSIS
bup margin [options...]
DESCRIPTION
bup margin iterates through all objects in your bup repository, calculating the largest number of prefix bits shared between any two
entries. This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.
For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45. That means a 46-bit
hash would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by
its first 46 bits.
The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects. Since SHA-1 hashes have 160 bits,
that leaves 115 bits of margin. Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits
with far fewer objects.
If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see if
you're getting dangerously close to 160 bits.
OPTIONS
--predict
Guess the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer
from the guess. This is potentially useful for tuning an interpolation search algorithm.
--ignore-midx
don't use .midx files, use only .idx files. This is only really useful when used with --predict.
EXAMPLE
$ bup margin
Reading indexes: 100.00% (1612581/1612581), done.
40
40 matching prefix bits
1.94 bits per doubling
120 bits (61.86 doublings) remaining
4.19338e+18 times larger is possible
Everyone on earth could have 625878182 data sets
like yours, all in one repository, and we would
expect 1 object collision.
$ bup margin --predict
PackIdxList: using 1 index.
Reading indexes: 100.00% (1612581/1612581), done.
915 of 1612581 (0.057%)
SEE ALSO
bup-midx(1), bup-save(1)
BUP
Part of the bup(1) suite.
AUTHORS
Avery Pennarun <apenwarr@gmail.com>.
Bup unknown- bup-margin(1)