This introductory guide serves as a handy reference for common regular expressions (regex) you can use with the grep command. If you’re just starting out or need a quick refresh, this cheat sheet will make it easy to get the hang of the essential regex patterns that go hand in hand with grep.

Basic Patterns

  • .: Matches any single character
    • Example: grep "h.t" file.txt matches “hat”, “hot”, “hit”, etc.
  • ^: Matches the start of a line
    • Example: grep "^The" file.txt matches lines starting with “The”
  • $: Matches the end of a line
    • Example: grep "end$" file.txt matches lines ending with “end”
  • *: Matches zero or more occurrences of the previous character
    • Example: grep "ca*t" file.txt matches “ct”, “cat”, “caat”, etc.

Character Classes

  • [abc]: Matches any single character in the set
    • Example: grep "[aeiou]" file.txt matches any vowel
  • [^abc]: Matches any single character not in the set
    • Example: grep "[^0-9]" file.txt matches any non-digit
  • [a-z]: Matches any single character in the range
    • Example: grep "[A-Z]" file.txt matches any uppercase letter
  • [a-zA-Z]: Matches any lowercase or uppercase letter
    • Example: grep "[a-zA-Z]" file.txt matches any letter

Note: Character classes are case-sensitive unless used with the -i flag.

Basic vs Extended Regular Expressions

  • Use grep -E for extended features such as +, ?, and |.
  • For basic usage, use grep without the -E flag, though it has more limited regex capabilities.

Extended Regex Features (use with grep -E)

  • +: Matches one or more occurrences of the previous character
    • Example: grep -E "ca+t" file.txt matches “cat”, “caat”, but not “ct”
  • ?: Matches zero or one occurrence of the previous character
    • Example: grep -E "colou?r" file.txt matches both “color” and “colour”
  • |: Alternation (OR)
    • Example: grep -E "cat|dog" file.txt matches lines containing either “cat” or “dog”

Quantifiers

  • {n}: Matches exactly n occurrences
    • Example: grep -E "a{3}" file.txt matches “aaa”
  • {n,}: Matches n or more occurrences
    • Example: grep -E "a{2,}" file.txt matches “aa”, “aaa”, etc.
  • {n,m}: Matches between n and m occurrences
    • Example: grep -E "a{2,4}" file.txt matches “aa”, “aaa”, “aaaa”

Special Characters

  • \: Escapes special characters
    • Example: grep "\." file.txt matches actual periods
  • \b: Matches word boundaries
    • Example: grep "\bthe\b" file.txt matches “the” but not “there” or “other”

Practical Examples

  1. Find all IP addresses: 
    grep -E "\b([0-9]{1,3}\.){3}[0-9]{1,3}\b" file.txt
  2. Match email addresses: 
    grep -E "[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}" file.txt
  3. Find lines with dates (YYYY-MM-DD format): 
    grep -E "^[0-9]{4}-[0-9]{2}-[0-9]{2}" file.txt
  4. Match lines starting with “Error” or “Warning”: 
    grep -E "^(Error|Warning)" log.txt
  5. Find words with at least eight characters: 
    grep -E "\b[A-Za-z]{8,}\b" file.txt
  6. Match lines that do not contain “Error” (inverse match): 
    grep -v "Error" log.txt
  7. Case-insensitive search for the word “hello”: 
    grep -i "hello" file.txt

Backslash Characters

When using grep -E or grep -P (PCRE mode), the following backslash characters have special meanings:

  • \b: Word boundary
    • Example: grep -E "\bcat\b" file.txt matches “cat” but not “category”
  • \B: Non-word boundary
    • Example: grep -E "\Bcat\B" file.txt matches “cat” in “concatenate” but not “cat” alone
  • \d: Any digit (equivalent to [0-9])
    • Example: grep -E "\d{3}" file.txt matches any three consecutive digits
  • \D: Any non-digit (equivalent to [^0-9])
    • Example: grep -E "\D+" file.txt matches one or more non-digit characters
  • \s: Any whitespace character (space, tab, newline)
    • Example: grep -E "hello\sworld" file.txt matches “hello world” with any whitespace between
  • \S: Any non-whitespace character
    • Example: grep -E "\S{5}" file.txt matches any five consecutive non-whitespace characters
  • \w: Any word character (alphanumeric + underscore, equivalent to [a-zA-Z0-9_])
    • Example: grep -E "\w+" file.txt matches one or more word characters
  • \W: Any non-word character
    • Example: grep -E "\W" file.txt matches any single non-word character
  • \\: Literal backslash
    • Example: grep -E "\\" file.txt matches a single backslash character

ℹ️ Note: When using basic grep (without -E or -P), you may need to escape these special backslash characters with an additional backslash, like \\s or \\d.

Practical Examples with Backslash Characters

  1. Find words starting with “pre”: 
    grep -E "\bpre\w+" file.txt
  2. Match phone numbers in format (xxx) xxx-xxxx: 
    grep -E "\(\d{3}\)\s?\d{3}-\d{4}" file.txt
  3. Find lines with exactly 3 words: 
    grep -E "^\s*\S+\s+\S+\s+\S+\s*$" file.txt
  4. Match email addresses using \w: 
    grep -E "\b[\w.%+-]+@[\w.-]+\.[A-Za-z]{2,}\b" file.txt