Search for special characters using grep

Question

I want to search for the lines that contains any of the following characters:

: / / ? # [ ] @ ! $ & ' ( ) * + , ; = %

I am familiar with grep. But with this set of characters, it is a bit tricky and I would rather ask for help.

Answer 1

grep "[]:/?#@\!\$&'()*+,;=%[]"

Within a bracketed expression, [...], very few character are "special" (only a very small subset, like ], - and ^, and the three combinations [=, [: and [.). When including ] in [...], the ] must come first (possibly after a ^). I opted to put the ] first and the [ last for symmetry.

The only other thing to remember is that a single quoted string can not include a single quote, so we use double quotes around the expression. Since we use a double quoted string, the shell will poke around in it for things to expand. For this reason, we escape the $ as \$ which will make the shell give a literal $ to grep, and we escape ! as \! too as it's a history expansion in bash (only in interactive bash shells though).

Would you want to include a backslash in the set, you would have to escape it as \\ so that the shell gives a single backslash to grep. Also, if you want to include a backtick `, it too must be escaped as \` as it starts a command substitution otherwise.

The command above would extract any line that contained at least one of the characters in the bracketed expression.

---

Using a single quoted string instead of a double quoted string, which gets around most of the annoyances with what characters the shell interprets:

grep '[]:/?#@!$&'"'"'()*+,;=%[]'

Here, the only thing to remember, apart from the placing of the ], is that a single quoted string can not include a single quote, so instead we use a concatenation of three strings:

1. '[]:/?#@!$&'
2. "'"
3. '()*+,;=%[]'

Answer 2

You can use [:punct:] character class if you don't mind that it also matches other punctuation and special characters:

grep '[[:punct:]]' file

Answer 3

You can use full regex to find special characters inside of square brackets if your looking for one character that is a special character. A great resource practicing, learning and checking your Regular Expression is regex101.com.

This uses Perl regular expressions, which can be used with GNU grep with the -P option:

grep -P "(\:|\/|\?|\#|\@|\!|\\$|\&|\'|\(|\)|\*|\+|\,|\;|\=|\%|\[|\])"
                            ^^^

Note that you need two backslashes in front of the dollar sign, as it has a special meaning in the shell, and the first backslash will escape it for the shell. ^{(With just one backslash in front, the shell would remove the backslash, grep would see an unescaped dollar sign meaning end of line, and match any input line.)}

If your terminal supports colors, throw colors on as well,

grep --color=auto -P "(\:|\/|\?|\#|\@|\!|\\$|\&|\'|\(|\)|\*|\+|\,|\;|\=|\%|\[|\])"

Here is the explanation of my regex from regex101.com

/(\:|\/|\?|\#|\@|\!|\$|\&|\'|\(|\)|\*|\+|\,|\;|\=|\%|\[|\])/gm
1st Capturing Group (\:|\/|\?|\#|\@|\!|\$|\&|\'|\(|\)|\*|\+|\,|\;|\=|\%|\[|\])
  \: matches the character : literally (case sensitive)
  \/ matches the character / literally (case sensitive)
  \? matches the character ? literally (case sensitive)
  \# matches the character # literally (case sensitive)
  \@ matches the character @ literally (case sensitive)
  \! matches the character ! literally (case sensitive)
  \$ matches the character $ literally (case sensitive)
  \& matches the character & literally (case sensitive)
  \' matches the character ' literally (case sensitive)
  \( matches the character ( literally (case sensitive)
  \) matches the character ) literally (case sensitive)
  \* matches the character * literally (case sensitive)
  \+ matches the character + literally (case sensitive)
  \, matches the character , literally (case sensitive)
  \; matches the character ; literally (case sensitive)
  \= matches the character = literally (case sensitive)
  \% matches the character % literally (case sensitive)
  \[ matches the character [ literally (case sensitive)
  \] matches the character ] literally (case sensitive)