Regex Cheatsheet

Introduction

Testing your regular expressions can be done in two ways:

1. Create a text file with some test paragraphs on a Unix machine, then use egrep <pattern> <file> to see what matches and what doesn't.
2. Use an online editor like regexr.com. Add your text in the "Text" field and type your expressions in the "Expression" field. This method is highly recommended for ease of use.

Charsets

When searching for specific strings or patterns in a file or block of text, regular expressions (regex) come in handy.

Basic Charsets

• [abc]: Matches any occurrence of 'a', 'b', or 'c'.
• [abc]zz: Matches 'azz', 'bzz', and 'czz'.

Ranges

• [a-c]zz: Same as above.
• [a-cx-z]zz: Matches 'azz', 'bzz', 'czz', 'xzz', 'yzz', and 'zzz'.
• [a-zA-Z]: Matches any single letter (lowercase or uppercase).
• file[1-3]: Matches 'file1', 'file2', and 'file3'.

Exclusions

• [^k]ing: Matches 'ring', 'sing', '$ing', but not 'king'.
• [^a-c]at: Matches 'fat' and 'hat', but not 'bat' or 'cat'.

Note that

1. Charsets match any occurrence of the specified characters, not just strings.
2. Type characters in the same order as the questions to avoid confusion.
3. Efficient regex means:
   • Be specific: Use [a-c] instead of [a-z] if only matching 'a' to 'c'.
   • Don't be too specific: Use [a-z] for a broader range to avoid complexity.

Examples

1. Match characters c, o, g:

   [cog]

2. Match words cat, fat, hat:

   [cfh]at

3. Match words Cat, cat, Hat, hat:

   [CcHh]at

4. Match filenames File1, File2, file3, file4, file5, File7, file9:

   [Ff]ile[1-9]

5. Match all filenames except "File7":

   [Ff]ile[^7]

Wildcards and Optional Characters

• .: Matches any single character except the line break.
• ?: Makes the preceding character optional.

Examples

1. Match words Cat, fat, hat, rat:

   .at

2. Match words Cat, cats:

   [Cc]ats?

3. Match domain name cat.xyz:

   cat\.xyz

4. Match domain names cat.xyz, cats.xyz, hats.xyz:

   [ch]ats?\.xyz

5. Match 4-letter strings not ending with letters n to z:

   ...[^n-z]

6. Match bat, bats, hat, hats, but not rat or rats (use the hat symbol)

   [^r]ats?

Metacharacters and Repetitions

Basic Metacharacters

• \d: Matches a digit.
• \D: Matches a non-digit.
• \w: Matches an alphanumeric character.
• \W: Matches a non-alphanumeric character.
• \s: Matches a whitespace character.
• \S: Matches everything else.

Repetitions

• {n}: Matches exactly n times.
• {n,m}: Matches between n and m times.
• {n,}: Matches n or more times.
• *: Matches 0 or more times.
• +: Matches 1 or more times.

Examples

1. Match word catssss:

   cats{4}

2. Match words Cat, cats, catsss:

   [Cc]ats*

3. Match sentences regex go br, regex go brrrrrr:

   regex go br+

4. Match filenames ab0001, bb0000, abc1000, cba0110, c0000:

   [abc]{1,3}[01]{4}

5. Match filenames File01, File2, file12, File20, File99:

   [Ff]ile\d{1,2}

6. Match all of the following folder names: kali tools, kali tools:

   kali\s+tools

7. Match all of the following filenames: notes~, stuff@, gtfob#, lmaoo!:

   \w{5}\W

8. Match the string in quotes (use the * sign and the \s, \S metacharacters): "2f0h@f0j0%! a)K!F49h!FFOK":

   \S*\s*\S*

9. Match every 9-character string (with letters, numbers, and symbols) that doesn't end in a "!" sign:

   \S{8}[^!]

10. Match all of these filenames (use the + symbol): .bash_rc, .unnecessarily_long_filename, and note1:

    \.?[\w_]+

Start/End Patterns, Groups, and Either/Or

Start/End Characters

• ^: Matches the start of a line.
• $: Matches the end of a line.

Groups and Either/Or

• (pattern): Defines a group.
• |: Specifies an either/or pattern.

Examples

1. Match string starting with "Password:" followed by 10 characters excluding "0":

   Password:[^0]{10}

2. Match "username: " at the beginning of a line:

   ^username:\s

3. Match lines not starting with a digit:

   ^\D

4. Match string EOF$ at the end of a line:

   EOF\$$

5. Match sentences:
   I use vim
I use nano

   I use (nano|vim)

6. Match lines starting with $ followed by a digit, another $, and non-whitespace characters:

   \$\d\$\S+

7. Match any IPv4 address:

   (\d{1,3}\.){3}\d{1,3}

8. Match emails hello@tryhackme.com, username@domain.com, dummy_email@xyz.com and group the username and domain:

   (\w+)@(\w+)\.com

Author

This cheatsheet is created by Purva Patel.