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回复 5# xiaogui_vip
https://www.gnu.org/software/gawk/manual/gawk.html#String-Functions
9.1.3 String-Manipulation Functions
The functions in this section look at or change the text of one or more strings.
....
sub(regexp, replacement [, target])
Search target, which is treated as a string, for the leftmost, longest substring matched by the regular expression regexp. Modify the entire string by replacing the matched text with replacement. The modified string becomes the new value of target. Return the number of substitutions made (zero or one).
The regexp argument may be either a regexp constant (/…/) or a string constant ("…"). In the latter case, the string is treated as a regexp to be matched. See Computed Regexps, for a discussion of the difference between the two forms, and the implications for writing your program correctly.
This function is peculiar because target is not simply used to compute a value, and not just any expression will do—it must be a variable, field, or array element so that sub() can store a modified value there. If this argument is omitted, then the default is to use and alter $0.47 For example:
str = "water, water, everywhere"
sub(/at/, "ith", str)
sets str to ‘wither, water, everywhere’, by replacing the leftmost longest occurrence of ‘at’ with ‘ith’.
If the special character ‘&’ appears in replacement, it stands for the precise substring that was matched by regexp. (If the regexp can match more than one string, then this precise substring may vary.) For example:
{ sub(/candidate/, "& and his wife"); print }
changes the first occurrence of ‘candidate’ to ‘candidate and his wife’ on each input line. Here is another example:
$ awk 'BEGIN {
> str = "daabaaa"
> sub(/a+/, "C&C", str)
> print str
> }'
-| dCaaCbaaa
This shows how ‘&’ can represent a nonconstant string and also illustrates the “leftmost, longest” rule in regexp matching (see Leftmost Longest).
The effect of this special character (‘&’) can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write ‘\\&’ in a string constant to include a literal ‘&’ in the replacement. For example, the following shows how to replace the first ‘|’ on each line with an ‘&’:
{ sub(/\|/, "\\&"); print }
As mentioned, the third argument to sub() must be a variable, field, or array element. Some versions of awk allow the third argument to be an expression that is not an lvalue. In such a case, sub() still searches for the pattern and returns zero or one, but the result of the substitution (if any) is thrown away because there is no place to put it. Such versions of awk accept expressions like the following:
sub(/USA/, "United States", "the USA and Canada")
For historical compatibility, gawk accepts such erroneous code. However, using any other nonchangeable object as the third parameter causes a fatal error and your program will not run.
Finally, if the regexp is not a regexp constant, it is converted into a string, and then the value of that string is treated as the regexp to match.
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发表于 2015-09-02 10:31
发表于 2015-09-02 10:50
