Introduction
Regular expressions, or regex, are sequences of characters that define a search pattern that is primarily used for pattern matching within strings. They are commonly used in validating, searching, replacing, or extracting specific patterns from strings. Common uses are finding or matching strings that match a required format like phone numbers, email addresses, or dates.
Character Classes
Character Class | Syntax | Description
--- |: --- :| ---
Character Set | [ABC] | Matches any character in the set
Negated Set | [^ABC] | Matches any character not in the set
Range | [A-Z] | Matches any characters in between the two specified characters, inclusively
Dot | . | Matches any characters except line breaks
Word | \w | Matches any word character (alphanumeric & underscore) Equivalent to [A-Za-z0-9_]
Not Word | \W | Matches any characters that are not a word character
Digit | \d | Matches any number (0-9)
Digit | \D | Matches any character that is not a number
Anchors
| Anchor | Syntax | Description |
|---|---|---|
| Beginning | ^ | Matches the beginning of the string |
| End | $ | Matches the end of the string |
| Word Boundary | \b | Matches a word boundary position between a word character and non-word character |
| Not Word Boundary | \B | Matches any position that is not a word boundary |
Reserved Characters
In regular expressions, some characters, (+ * ? ^ $ \ . [ ] { } ( ) | /) are reserved for specific purposes, as seen above. To represent these characters as a literal character, they should be preceded by a backslash ().
Quantifiers & Alternation
| Syntax | Description | |
|---|---|---|
| Plus | + | Matches 1 or more of the preceding token |
| Star | * | Matches 0 or more of the preceding token |
| Quantifier | {1,3} | Matches the specified quantity of the preceding token. {1,3} will match 1 to 3 and {3} will match exactly 3. {3,} will match 3 or more. |
| Optional | ? | Matches 0 or 1 of the preceding token |
| Lazy | ? | Makes the preceding quantifier lazy, making it match as few characters as possible |
| Alternation | | | Matches the expression before or after the | |
Examples
| Example | Syntax |
|---|---|
| Phone Number | ^(\d{3})\s?\d{3}[-\s]?\d{4}$ |
| Email Address | ^\w+@[a-zA-Z_]+?.[a-zA-Z]{2,3}$ |
| Date (MM/DD/YY) | ^(0[1-9]|1[0-2])/(0[1-9]|[12]\d|3[01])/(\d{2})$ |
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