Regex

Question 1

How to find a plain string?

Answer 1

Just use the string.
i.e. abc finds abc in a file

Note that grep finds each line that contains this.

Question 2

How to include spaces in a regex search?

Answer 2

You need to need single or double quotation marks.

Question 3

Anchoring in regex

Answer 3

Use ^ to match for the beginning.
i.e.
^abc finds strings that start with abc

Use $ to match for the end
i.e.
abc$ finds strings that end with abc

Note that these can be combined.
i.e.
^abc$ finds lines that only contain abc

Question 4

How to regex for empty lines?

Answer 4

^$

Question 5

How to match a single character?

Answer 5

Use .
For example:
m..t
could find malt, meet, moot…

Question 6

What is a bracket expression?

Answer 6

Allow matching of characters by enclosing them in []
For example:
acce[np]t would fines lines containing accent and accept

Question 7

How to do a bracket expression that excludes certain characters?

Answer 7

Use ^ at the beginning of the expression.
i.e.
co[^l]a
would find coca, cobalt, but not cola.

Question 8

How to specify a range of characters?

Answer 8

Inside of the brackets, use a - to connect the beginning and end.
i.e.
[1-9] is equivalent to [123456789]
[a-e] is equivalent to [abcde]

Question 9

How to find some type/group of characters?

Answer 9

Use the predefined classes.
I.e.
[:alnum:]

Question 10

Alphanumeric class

Answer 10

:alnum:

Question 11

Alphabetic class

Answer 11

:alpha:

Question 12

Spaces and tabs class

Answer 12

:blank:

Question 13

Digit class

Answer 13

:digit:

Question 14

Lowercase letters class

Answer 14

:lower:

Question 15

Uppercase letters class

Answer 15

:upper:

Question 16

What is a quantifier?

Answer 16

Allows you to specify the number of occurrences that must be present to match.
*
?
+
{n}
{n,}
{,m}
{n,m}
Note that this works on the PRECEEDING character.

Question 17

How to match 0 or more times?

Answer 17

*

Question 18

How to match 0 or 1 time?

Answer 18

?

Question 19

How to match 1 or more times?

Answer 19

+

Question 20

How to match exactly n times?

Answer 20

{n}

Question 21

How to match at least n times?

Answer 21

{n,}

Question 22

How to match at most m times?

Answer 22

{,m}

Question 23

How to match n to m times?

Answer 23

{n,m}

Question 24

How to include a regex symbol as a string so that it is not interpreted as regex?

Answer 24

Use an escape character.

Question 25

How to match different possible matches?

Answer 25

Use |
For example, ‘big|large|massive’ matches big, OR large, OR massive.

Note that using -E for extended regular expressions, \ can be omitted (don’t escape |).

Question 26

How to group things?

Answer 26

Use brackets. Note, must use \ on parenthesis when not using extended regex.
For example ‘(fear)?less’ finds lines that include less preceded by 1 or 0 fear.

Question 27

Special backslash expressions.

Answer 27

\b matches a word boundary
< matches an empty string at the beginning of a word
> matches an empty string at the end of a word
\w matches a word
\s matches a space

For example,
‘\bhello\b’ finds hello, but not if it is embedded in a larger word.