In this book Title:Unix Power Tools, Third Edition
URL:
Amazon.com: Unix Power Tools, Third Edition: Shelley Powers, Jerry Peek, Tim O'Reilly, Mike Loukides: Books
ISBN:0596003307
Author:Shelley Powers / Jerry Peek / Tim O'Reilly / Mike Loukides
Publisher:O'Reilly & Associates
Page:1200 pages
Edition:3rd edition (October 1, 2002)
32.13 Regular Expressions: Remembering Patterns with \ (, \ ), and \1
Another pattern that requires a special mechanism is searching for repeated words. The expression [a-z][a-z] will match any two lowercase letters. If you wanted to search for lines that had two adjoining identical letters, the above pattern wouldn't help. You need a way to remember what you found and see if the same pattern occurs again. In some programs, you can mark part of a pattern using \( and \). You can recall the remembered pattern with \ followed by a single digit.[4] Therefore, to search for two identical letters, use \([a-z]\)\1. You can have nine different remembered patterns. Each occurrence of \( starts a new pattern. The regular expression to match a five-letter palindrome (e.g., "radar") is: \([a-z]\)\([a-z]\)[a-z]\2\1. [Some versions of some programs can't handle \( \) in the same regular expression as \1, etc. In all versions of sed, you're safe if you use \( \) on the pattern side of an s command — and \1, etc., on the replacement side (Section 34.11). — JP]
— BB
34.11 Referencing Portions of a Search String
In sed, the substitution command provides metacharacters to select any individual portion of a string that is matched and recall it in the replacement string. A pair of escaped parentheses are used in sed to enclose any part of a regular expression and save it for recall. Up to nine "saves" are permitted for a single line. \n is used to recall the portion of the match that was saved, where n is a number from 1 to 9 referencing a particular "saved" string in order of use. (Section 32.13 has more information.)
For example, when converting a plain-text document into HTML, we could convert section numbers that appear in a cross-reference into an HTML hyperlink. The following expression is broken onto two lines for printing, but you should type all of it on one line:
s/\([sS]ee \)\(Section \)\([1-9][0-9]*\)\.\([1-9][0-9]*\)/
\1<a href="#SEC-\3_\4">\2\3.\4<\/a>/
Four pairs of escaped parentheses are specified. String 1 captures the word see with an upper- or lowercase s. String 2 captures the section number (because this is a fixed string, it could have been simply retyped in the replacement string). String 3 captures the part of the section number before the decimal point, and String 4 captures the part of the section number after the decimal point. The replacement string recalls the first saved substring as \1. Next starts a link where the two parts of the section number, \3 and \4, are separated by an underscore (_) and have the string SEC- before them. Finally, the link text replays the section number again — this time with a decimal point between its parts. Note that although a dot (.) is special in the search pattern and has to be quoted with a backslash there, it's not special on the replacement side and can be typed literally. Here's the script run on a short test document, using checksed (Section 34.4):
% checksed testdoc
********** < = testdoc > = sed output **********
8c8
< See Section 1.2 for details.
---
> See <a href="#SEC-1_2">Section 1.2</a> for details.
19c19
< Be sure to see Section 23.16!
---
> Be sure to see <a href="#SEC-23_16">Section 23.16</a>!
We can use a similar technique to match parts of a line and swap them. For instance, let's say there are two parts of a line separated by a colon. We can match each part, putting them within escaped parentheses and swapping them in the replacement:
% cat test1
first:second
one:two
% sed 's/\(.*\):\(.*\)/\2:\1/' test1
second:first
two
ne
The larger point is that you can recall a saved substring in any order and multiple times. If you find that you need more than nine saved matches, or would like to be able to group them into matches and submatches, take a look at Perl.
Section 43.10, Section 31.10, Section 10.9, and Section 36.23 have examples.
—DD and JP
I test it it works for a list of lines in the same pattern. The problem in my situation is that I fail to in the first step put all the content of this regular expression <w c5="(.*?)" hw="(.*?)" pos="(.*?)">(.*?)<\/w>/in each individual line such as <w c5="NN1" hw="factsheet" pos="SUBST">FACTSHEET</w>
<w c5="DTQ" hw="what" pos="PRON">WHAT</w>
<w c5="VBZ" hw="be" pos="VERB">IS</w>
<w c5="NN1" hw="aids" pos="SUBST">AIDS</w>
My result is not that clear which contains other contents out of the regular expression such as <s n="420">.
To my strange, it works in that book's example but not in my situation.
Best
John
12-14-2008
