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Full Discussion: Delete special characters
Top Forums Shell Programming and Scripting Delete special characters Post 303015185 by bakunin on Thursday 29th of March 2018 05:57:34 PM
Old 03-29-2018
Quote:
Originally Posted by kenshinhimura
But pelase educate me, when to use a backslash to negate a special character?
The rule is: when you use a character that has a special meaning (to sed) and you want it to mean just the character itself you need to "escape" it - that is, prepend it with a backslash.

An example:

Code:
sed 's/abc./xyz/' /input/file

The dot (".") here doesn't mean a dot, but is a special character, meaning "any one character". If you want it to mean a real, literal dot and nothing else, you need to escape it:

Code:
sed 's/abc\./xyz/' /input/file

Notice that this escaping is implicit in some situations, for instance in "character-classes". Look at the following regular expression:

Code:
sed 's/a[bcd]e/xyz/' /input/file

This searches for an "a", followed by either an "b", a "c" or a "d", followed by an "e". It would match any of these strings:

abe
ace
ade

Inside the brackets all characters lose their special meaning. I.e. a[bc.]d would search for any of these strings:

abd
acd
a.d

If you would try to escape a character here the escaping backslash would be treated as a normal character too. The regexp a[bc\.]d would find these strings:

abd
acd
a\d
a.d

Notice that the characters you called special - "<", ">", etc. - don't have any special meaning in sed anyway and would have been safe to use without any escaping in first place.

I hope this helps.

bakunin
 

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MATCH(1L)						      Schily's USER COMMANDS							 MATCH(1L)

NAME
match - searches for patterns in files SYNOPSIS
match [ -option ] pattern [ file ... ] DESCRIPTION
Match searches the named files or standard input (if no filenames are given) for the occurrences of the given pattern on each line. The program accepts literal characters or special pattern matching characters. All lines that match the pattern are output on standard output. You can only specify one pattern string for each match, however, you can construct an arbitrarily complex string. When you do not specify a file, match can be used as a filter to display desired lines. Standard in is used if no files are specified. OPTIONS
-not, -v Prints all lines that do not match. -i Ignore the case of letters -m Force not to use the magic mode -w Search for pattern as a word -x Display only those lines which match exactly -c Display matching count for each file -l Display name of each file which matches -s Be silent indicate match in exit code -h Do not display filenames -n Precede matching lines with line number (with respect to the input file) -b Precede matching lines with block number REGULAR EXPRESSIONS
The following is a table of all the pattern matching characters: c An ordinary character (not one of the special characters discussed below) is a one character regular expression that matches that character. c A backslash () followed by any special character is a one character regular expression that matches the special character itself. The special characters are: ! # % * { } [ ] ? ^ $ ! Logical OR as in match this!that!the_other. You may have to use `{}' for precedence grouping. # A hash mark followed by any regular expression matches any number (including zero) occurrences of the regular expression. ? Matches exactly any one character. W? matches Wa, Wb, Wc, W1, W2, W3 ... * Matches any number of any character. % Matches exactly nothing. It can be used in groups of ored patterns to specify that an empty alternative is possible. {} Curly brackets may be used to enclose patterns to specify a precedence grouping, and may be nested. {%!{test}}version matches the strings testversion and version. [string] A non empty string of characters enclosed in square brackets is a one character regular expression that matches any one character in that string. If however the first character of the string is a circumflex (^), the one character expression matches any character which is not in the string. The ^ has this special meaning only if it occurs first in the string. The minus (-) may be used to indi- cate a range of consecutive ASCII characters; for example, [0-9] is equivalent to any one of the digits. The - loses it's special meaning if it occurs first (after an initial ^, if any) or last in the string. The right square bracket (]) and the backslash () must be quoted with a backslash if you want to use it within the string. ^ Matches the beginning of a line. $ Matches the end of a line. (^*$ matches any entire line) EXAMPLES
FILES
None. SEE ALSO
grep(1), fgrep(1), egrep(1) DIAGNOSTICS
NOTES
Even if a match occurs more than once per line, the line is output only once. Quote special pattern matching characters to prevent them from being expanded by the Command Interpreter. BUGS
The length of the pattern is currently limited to 100 characters. This limit is reduced by 38 if the -w option is used. Joerg Schilling 15. Juli 1988 MATCH(1L)
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