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I found this question really relevant to what I wanted: Parsing using awk or sed in Unix, however I can't figure out what the following does:
's/\([,=]\) /\1/g'
I know that g does a global substitution but really can't understand what's going on within the context of the question.
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It means that sed will read the next line and start processing it. nothing will be printed other than the blank lines, three times each. Show activity on this post. The command d is only applied to empty lines here: '/^$/d;p;p'.
In some versions of sed, the expression must be preceded by -e to indicate that an expression follows. The s stands for substitute, while the g stands for global, which means that all matching occurrences in the line would be replaced.
The syntax of the s command is ' s/ regexp / replacement / flags '. Its basic concept is simple: the s command attempts to match the pattern space against the supplied regular expression regexp ; if the match is successful, then that portion of the pattern space which was matched is replaced with replacement .
The above sed command replaces the string only on the third line. Duplicating the replaced line with /p flag : The /p print flag prints the replaced line twice on the terminal. If a line does not have the search pattern and is not replaced, then the /p prints that line only once.
Here's a simple example:
$ echo 'abcabcabc' | sed 's/\(ab\)c/\1/' ababcabc $ echo 'abcabcabc' | sed 's/\(ab\)c/\1/g' ababab $ echo 'abcabcabc' | sed 's/\(ab\)\(c\)/\1d\2/g' abdcabdcabdc In the first command, only the first match is affected. In the second command, every match is affected. In both cases, the \1 refers to the characters captured by the escaped parentheses.
In the third command, two capture groups are specified. They are referred to by using \1 and \2. Up to nine capture groups can be used.
In addition to the g (global) operator (or without it, the first match), you can specify a particular match:
$ echo 'aaaaaa' | sed 's/a/A/4' aaaAaa \(...\) would capture the characters specified inside of the parens and \1 would be used to reference the first match, this is a part of regex.
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