Regular Expressions (Regex)

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Regular expression (regex) is a tool for expressing patterns in text.

• a group of characters that describe how to execute a specific search pattern on a given text
• two forms:
  • basic
  • extended
• programs determine which form is supported
  • differences are complex and subtle
• use cases:
  • validate data
  • find patterns in large amounts of text
  • search and replace text
  • validate email addresses
  • search for phone numbers

Element	Purpose
[ABC]	Character set
[A-Z]	Range
\w	Word
\d	Digit
\s	Whitespace
^	Beginning
$	End
?	May or may not exist
{1,3}	Quantifier

Basic Features

Bracket Expressions

Bracket expressions utilize characters enclosed in brackets [] which match any one character within the brackets.

• e.g., b[aeiou]g matches bag, beg, big, bog, and bug
• brackets represent a single character in the word
• using a caret ^ after the opening bracket matches against any character except the ones specified
  • e.g., b[^aeiou]g matches bbg or bAg, but not bag or beg

Range Expressions

A range expression is a variant on a bracket expression that uses a range of start and end points separated by a dash -.

• e.g., a[2-4]z matches a2z. a3z, and a4z

Any Single Character

The dot . represents any single character except a newline.

• e.g, a.z matches a2z, abz, etc.

Start and End of Line

A text line (aka a record) consists of all the characters before the line is terminated with a newline.

• caret ^ represents the start of a line
  • when not used inside of brackets
• dollar sign $ represents the end of a line

Repetition

A full or partial regular expression may be followed by a special symbol to denote repetition of the matched item.

• an asterisk * denotes zero or more matches
• often combined with dot .* to specify a match with any substring
  • e.g., A.*Lincoln matches Abe Lincoln and Abraham Lincoln

Escaping

To match a special character literally, you need to escape it.

• precede the character with a backslash \
• e.g., to match www.test.com need to do www\.test\.com

Extended Features

Additional Repetition Operators

A plus sign + matches one or more occurrences.

A question mark ? matches zero or one match.

Multiple Possible Strings

The vertical bar | separates two possible matches.

• e.g., car|truck matches either car or truck

Parentheses

Parentheses () surround subexpressions.

• often used to specify how to apply operators
• e.g., file(one|two|three)\.txt

Grep with Regular Expressions

• to use an extended regular expression with grep, you need to include the -E option