Regex
charsets
defined by enclosing in [] the characters or range of characters you want to match. Then it finds every occurance of the pattern you have defined.
[abc]
will match a, b, and c (every occurrence of each letter)
[abc]zz
will match azz, bzz, and czz.
You can also use a - dash to define ranges:
[a-c]zz
is the same as above.
And then you can combine ranges together:
[a-cx-z]zz
will match azz, bzz, czz, xzz, yzz, and zzz.
Most notably, this can be used to match any alphabetical character:
[a-zA-Z]
will match any single letter (lowercase or uppercase).
You can use numbers too:
file[1-3]
will match file1, file2, and file3.
Then, there is a way to exclude characters from a charset with the ^ hat symbol, and include everything else.
[^k]ing
will match ring, sing, $ing, but not king.
Of course, you can exclude charsets, not just single characters.
[^a-c]at
will match fat and hat, but not bat or cat.
The wildcard that is used to match any single character (except the line break) is the . dot.
a.c
will match aac, abc, a0c, a!c, and so on.
Also, you can set a character as optional in your pattern using the ? question mark.
abc?
will match ab and abc, since the c is optional.
If you want to search for . a literal dot, you have to escape it with a reverse slash.
a.c
will match a.c, but also abc, a@c, and so on.
a.c
will match just a.c
\d is used to match any single digit. Here's a reference:
\d matches a digit, like 9
\D matches a non-digit, like A or @
\w matches an alphanumeric character, like a or 3
\W matches a non-alphanumeric character, like ! or #
\s matches a whitespace character (spaces, tabs, and line breaks)
\S matches everything else (alphanumeric characters and symbols)
Note: Underscores _ are included in the \w metacharacter and not in \W. That means that \w will match every single character in test_file.
Often we want a pattern that matches many characters of a single type in a row, and we can do that with repetitions. For example, {2} is used to match the preceding character (or metacharacter, or charset) two times in a row. That means that z{2} will match exactly zz.
Here's a reference for each repetition along with how many times it matches the preceding pattern:
{12} - exactly 12 times.
{1,5} - 1 to 5 times.
{2,} - 2 or more times.
* - 0 or more times.
+ - 1 or more times.
Sometimes it's very useful to specify that we want to search by a certain pattern in the beginning or the end of a line. We do that with these characters:
^ - starts with
$ - ends with
So for example, if you want to search for a line that starts with abc, you can use ^abc.
If you want to search for a line that ends with xyz, you can use xyz$.
Note: The ^ hat symbol is used to exclude a charset when enclosed in [square brackets], but when it is not, it is used to specify the beginning of a word.
You can also define groups by enclosing a pattern in (parentheses). This function can be used for many ways that are not in the scope of this tutorial. We will use it to define an either/ or pattern, and also to repeat patterns. To say "or" in Regex, we use the | pipe.
For an "either/or" pattern example, the pattern during the (day|night) will match both of these sentences: during the day and during the night.
For a repetition example, the pattern (no){5} will match the sentence nonononono.