05-02-2014
Quote:
Originally Posted by
alister
That code is vulnerable to pattern matching metacharacters. For this approach to work with arbitrary text, it is necessary to double-quote the nested parameter expansion.
You are right and your example is legitimate. I left that part out purposefully to avoid complicating matters. I should have probably mentioned it.
Quote:
Originally Posted by
alister
A minor nit: You refer to shell pattern matching as regular expressions. I'm sure you know that those are two distinct grammars, but a novice may become confused.
Yes - and no. "regular expressions" is (in a very theoretical sense) any type-3 language in the Chomsky hierarchy: a device where some characters and some metacharacters describe a text pattern. This is the case for shell regexps (aka "file globs") as well as for "Unix Basic Regular Expressions" (what awk, grep and sed use) or "Extended Regular Expressions" (i.e. perl and some GNU variants of grep, sed, ...). These are all different flavours of Regexps (and i should have mentioned that too, probably), but still Regexps nevertheless.
You are right, though, that in UNIX environments, the term "regexp" particularily describes BREs as used in sed, awk and grep. Every other use of the term, even if technically correct, might be confusing.
bakunin
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GLOB(7) BSD Miscellaneous Information Manual GLOB(7)
NAME
glob -- shell-style pattern matching
DESCRIPTION
Globbing characters (wildcards) are special characters used to perform pattern matching of pathnames and command arguments in the csh(1),
ksh(1), and sh(1) shells as well as the C library functions fnmatch(3) and glob(3). A glob pattern is a word containing one or more unquoted
'?' or '*' characters, or ``[..]'' sequences.
Globs should not be confused with the more powerful regular expressions used by programs such as grep(1). While there is some overlap in the
special characters used in regular expressions and globs, their meaning is different.
The pattern elements have the following meaning:
? Matches any single character.
* Matches any sequence of zero or more characters.
[..] Matches any of the characters inside the brackets. Ranges of characters can be specified by separating two characters by a '-' (e.g.
``[a0-9]'' matches the letter 'a' or any digit). In order to represent itself, a '-' must either be quoted or the first or last
character in the character list. Similarly, a ']' must be quoted or the first character in the list if it is to represent itself
instead of the end of the list. Also, a '!' appearing at the start of the list has special meaning (see below), so to represent
itself it must be quoted or appear later in the list.
Within a bracket expression, the name of a character class enclosed in '[:' and ':]' stands for the list of all characters belonging
to that class. Supported character classes:
alnum cntrl lower space
alpha digit print upper
blank graph punct xdigit
These match characters using the macros specified in ctype(3). A character class may not be used as an endpoint of a range.
[!..] Like [..], except it matches any character not inside the brackets.
Matches the character following it verbatim. This is useful to quote the special characters '?', '*', '[', and '' such that they
lose their special meaning. For example, the pattern ``\*[x]?'' matches the string ``*[x]?''.
Note that when matching a pathname, the path separator '/', is not matched by a '?', or '*', character or by a ``[..]'' sequence. Thus,
/usr/*/*/X11 would match /usr/X11R6/lib/X11 and /usr/X11R6/include/X11 while /usr/*/X11 would not match either. Likewise, /usr/*/bin would
match /usr/local/bin but not /usr/bin.
SEE ALSO
fnmatch(3), glob(3), re_format(7)
HISTORY
In early versions of UNIX, the shell did not do pattern expansion itself. A dedicated program, /etc/glob, was used to perform the expansion
and pass the results to a command. In Version 7 AT&T UNIX, with the introduction of the Bourne shell, this functionality was incorporated
into the shell itself.
BSD
November 30, 2010 BSD