Regex matches but sed fails replace - regex

I am having a tricky regex issue
I have the string like below
some_Name _ _Bday Date Comm.txt
And here is my regex to match the spaces and underscore
\_?\s\_?
Now when i try to replace the string using sed and the above regex
echo "some_Name _ _Bday Date Comm.txt" | sed 's/\_?\s\_?/\_/g'
The output i want is
some_Name_Bday_Date_Comm.txt
Any ideas on how do i go about this ?

You are using a POSIX BRE regex engine with the \_?\s\_? pattern that matches a _?, a whitespace (if your sed supports \s shorthand) an a _? substring, i.e. the ? are treated as literal question mark symbols.
You may use
sed -E 's/[[:space:]_]+/_/g'
sed 's/[[:space:]_]\{1,\}/_/g'
See online sed demo
The [[:space:]_]+ POSIX ERE pattern (enabled with -E option) will match one or more whitespace or underscore characters.
The POSIX ERE + quantifier can be written as \{1,\} in POSIX BRE. Also, if you use a GNU sed, you may use \+ in the second sed command.

This might work for you (GNU sed):
sed -E 's/\s(\s*_)*/_/g' file
This will replace a space followed by zero or more of the following: zero or more spaces followed by an underscore.

Related

Bash script to enclose words in single quotes

I'm trying to write a bash script to enclose words contained in a file with single quotes.
Word - Hello089
Result - 'Hello089',
I tried the following regex but it doesn't work. This works in Notepad++ with find and replace. I'm not sure how to tweak it to make it work in bash scripting.
sed "s/(.+)/'$1',/g" file.txt > result.txt
Replacement backreferences (also called placeholders) are defined with \n syntax, not $n (this is perl-like backreference syntax).
Note you do not need groups here, though, since you want to wrap the whole lines with single quotation marks. This is also why you do not need the g flags, they are only necessary when you plan to find multiple matches on the same line, input string.
You can use the following POSIX BRE and ERE (the one with -E) solutions:
sed "s/..*/'&',/" file.txt > result.txt
sed -E "s/.+/'&',/" file.txt > result.txt
In the POSIX BRE (first) solution, ..* matches any char and then any 0 or more chars (thus emulating .+ common PCRE pattern). The POSIX ERE (second) solution uses .+ pattern to do the same. The & in the right-hand side is used to insert the whole match (aka \0). Certainly, you may enclose the whole match with capturing parentheses and then use \1, but that is redundant:
sed "s/\(..*\)/'\1',/" file.txt > result.txt
sed -E "s/(.+)/'\1',/" file.txt > result.txt
See the escaping, capturing parentheses in POSIX BRE must be escaped.
See the online sed demo.
s="Hello089";
sed "s/..*/'&',/" <<< "$s"
# => 'Hello089',
sed -E "s/.+/'&',/" <<< "$s"
# => 'Hello089',
$1 is expanded by the shell before sed sees it, but it's the wrong back reference anyway. You need \1. You also need to escape the parentheses that define the capture group. Because the sed script is enclosed in double quotes, you'll need to escape all the backslashes.
$ echo "Hello089" | sed "s/\\(.*\\)/'\1',/g"
'Hello089',
(I don't recall if there is a way to specify single quotes using an ASCII code instead of a literal ', which would allow you to use single quotes around the script.)

substitute single quotes in sed and perl

Could someone please explain what was happening with these two commands? Why do sed and perl give different results running the same regular expression pattern:
# echo "'" | sed -e "s/\'/\'/"
''
# echo "'" | perl -pe "s/\'/\'/"
'
# sed --version
sed (GNU sed) 4.5
You're using GNU sed, right? \' is an extension that acts as an anchor for end-of-string in GNU's implementation of basic regular expressions. So you're seeing two quotes in the output because the s matches the end of the line and adds a quote, after the one that was already in the line.
To make it a bit more obvious:
echo foo | sed -e "s/\'/#/"
produces
foo#
Documented here, and in the GNU sed manual
Edit: The equivalent in perl is \Z (or maybe \z depending on how you want to handle a trailing newline). Since \' isn't a special sequence in perl regular expressions, it just matches a literal quote. As mentioned in the other answer and comments, escaping a single quote inside a double quoted string isn't necessary, and as you've found, can potentially result in unintended behavior.

Regex with sed to search in files

I want to search recursiv in files for a given pattern and replace them. The search is for a string like "['DB']['1']['HOST'] = 'localhost'". If testing the regex the following doesn't print anything. Can't see an error in this regex? Could anyone help?
sed -n '/\[\'HOST\'\]\s?=\s?(?:\'|")(.+)(?:\'|")/p' /path/to/file
POSIX regex does not support non-capturing groups. Besides, you have not specified the -E option and the pattern is parsed as a BRE POSIX pattern where the capturing parentheses should be escaped. Also, the single quotes cannot be escaped to be used in a sed regex pattern, use \x27 instead.
Use
sed -En '/\[\x27HOST\x27\]\s?=\s?[\x27"][^\x27"]+[\x27"]/p'
See an online demo:
s="a string like ['DB']['1']['HOST'] = 'localhost'."
sed -En '/\[\x27HOST\x27\]\s?=\s?[\x27"][^\x27"]+[\x27"]/p' <<< "$s"
Besides, instead of \s, it might be a good idea to use [[:space:]].

Grep hashes via regex in bash

I want to grep for hexadecimal hashes in strings and only extract those hashes.
I've tested a regex in online regex testing tools that does the trick:
\b[0-9a-f][0-9a-f]+[0-9a-f]\b
The \b is used to set word boundaries (start & end) that should be any character 0-9 or a-f. Since I do not know if the hashes are 128bit or higher, I do not know the length of the hashes in advance. Therefore I set [0-9a-f]+ in the middle in order match any number of [0-9a-f], but at least one (since no hash consists just of two characters that are checked with the boundaries \b).
However, I noticed that
grep --only-matching -e "\b[0-9a-f][0-9a-f]+[0-9a-f]\b"
does not work in the shell, while the regex \b[0-9a-f][0-9a-f]*[0-9a-f]\b works in online regex testing tools.
In fact, the shell version does only work if I escape the quantifier + with a backslash:
grep --only-matching -e "\b[0-9a-f][0-9a-f]\+[0-9a-f]\b"
^
|_ escaped +
Why does grep needs this escaping in the shell?
Is there any downside of my rather simple approach?
I don't know why a metacharacter would need to be escaped in the bash, but your regex could be rewritten as this:
grep --only-matching -e "\b[0-9a-f]{3,}\b"
The + quantifier is not part of the POSIX Basic Regular Expressions (aka BRE) so you must escape it with grep in BRE mode.
As an alternative, you can:
add the -E flag to grep:
grep -E --only-matching -e "\b[0-9a-f][0-9a-f]+[0-9a-f]\b"
use [0-9a-f][0-9a-f]* or [0-9a-f]{1,}
Grep runs basic regular expressions by default. You need to escape the + quantifier with a backslash as it is said in the documentation:
In basic regular expressions the meta-characters ?, +, {, |,
(, and ) lose their special meaning; instead use the backslashed
versions \?, \+, \{, \|, \(, and \).
Also, there is no need for -e option, just
grep -o '\b[0-9a-f]\+\b' file

Pattern matching digits does not work in egrep?

Why can't I match the string
"1234567-1234567890"
with the given regular expression
\d{7}-\d{10}
with egrep from the shell like this:
egrep \d{7}-\d{10} file
?
egrep doesn't recognize \d shorthand for digit character class, so you need to use e.g. [0-9].
Moreover, while it's not absolutely necessary in this case, it's good habit to quote the regex to prevent misinterpretation by the shell. Thus, something like this should work:
egrep '[0-9]{7}-[0-9]{10}' file
See also
egrep mini tutorial
References
regular-expressions.info/Flavor comparison
Flavor note for GNU grep, ed, sed, egrep, awk, emacs
Lists the differences between grep vs egrep vs other regex flavors
For completeness:
Egrep does in fact have support for character classes. The classes are:
[:alnum:]
[:alpha:]
[:cntrl:]
[:digit:]
[:graph:]
[:lower:]
[:print:]
[:punct:]
[:space:]
[:upper:]
[:xdigit:]
Example (note the double brackets):
egrep '[[:digit:]]{7}-[[:digit:]]{10}' file
you can use \d if you pass grep the "perl regex" option, ex:
grep -P "\d{9}"
Use [0-9] instead of \d. egrep doesn't know \d.
try this one:
egrep '(\d{7}-\d{10})' file