Regex Basics: A Complete Beginner's Guide
What is a Regular Expression?
A regular expression (regex) is a sequence of characters that defines a search pattern. Regex is used for string matching, validation, and text manipulation across virtually all programming languages.
Basic Literal Matching
The simplest regex is just literal text. The pattern hello matches the string "hello" exactly.
`` Pattern: hello Text: Say hello to regex Match: "hello" at position 4
`
Metacharacters
Metacharacters have special meanings in regex:
The Dot (.)
Matches any single character except newline:
` Pattern: h.t Matches: "hat", "hot", "hit", "h@t"
`
Character Classes []
Match any character within the brackets:
` Pattern: [aeiou] Matches: any vowel Pattern: [0-9] Matches: any digit Pattern: [a-zA-Z] Matches: any letter
`
Negated Classes [^]
Match anything NOT in the brackets:
` Pattern: [^0-9] Matches: any non-digit character
`
Quantifiers
Control how many times a pattern matches:
Common Quantifiers
- *
- Zero or more times - +
- One or more times - ?
- Zero or one time - {n}
- Exactly n times - {n,}
- n or more times - {n,m}
- Between n and m times
` Pattern: a+ Matches: "a", "aa", "aaa", etc. Pattern: \d{3} Matches: exactly 3 digits like "123"
`
Anchors
Match positions rather than characters:
- ^
- Start of string/line - $
- End of string/line - \b
- Word boundary
` Pattern: ^Hello Matches: "Hello" only at the start Pattern: world$ Matches: "world" only at the end
`
Shorthand Character Classes
- \d
- Any digit [0-9] - \D
- Any non-digit - \w
- Any word character [a-zA-Z0-9_] - \W
- Any non-word character - \s
- Any whitespace - \S
- Any non-whitespace
Your First Regex
Let's validate an email (simplified):
` Pattern: \w+@\w+\.\w+ Breakdown:
``
Practice Tips