Regular Expression - Advanced Features

Advanced Features

Lookaheads and Lookbehinds

Lookahead

Pattern	Meaning
(?=...)	Positive lookahead
(?!...)	Negative lookahead

Examples:

\d+(?=px) # Matches digits followed by "px": "100px" → captures "100"
\d+(?!px) # Matches digits NOT followed by "px"

Lookbehind

Pattern	Meaning
(?<=...)	Positive lookbehind
(?<!...)	Negative lookbehind

Examples:

(?<=\$)\d+ # Matches digits preceded by $: "$100" → captures "100"
(?<!\\)prod # Matches "prod" NOT preceded by backslash

Negative Lookbehind Example

(?<!\\)prod

In plain language: Matches “prod” only when it is NOT preceded by a backslash.

Examples:

• “Let’s calculate the prod of these numbers” → “prod” would be matched
• “Use the \prod symbol for product notation” → “prod” would NOT be matched

The negative lookbehind ensures capturing instances of “prod” that aren’t part of a command or escaped sequence.

What is an Escaped Sequence?

An escaped sequence is a character preceded by a backslash (\) that has special meaning:

• \n - newline
• \t - tab
• \\ - literal backslash
• \prod - LaTeX command for product symbol (∏)

The backslash “escapes” the normal meaning of the following character.

Backreferences

Reference previously captured groups:

(\w+)\s+\1 # Matches repeated words: "hello hello"
(['"]).*?\1 # Matches quoted strings: "hello" or 'world'

• \1 refers to first captured group
• \2 refers to second captured group
• etc.

Greedy Vs Lazy Quantifiers

Greedy	Lazy	Behavior
*	*?	Match 0+ (lazy)
+	+?	Match 1+ (lazy)
?	??	Match 0-1 (lazy)
{n,m}	{n,m}?	Match n-m (lazy)

Example:

<div>Hello</div><div>World</div>

<div>.*</div> # Greedy: Matches entire string
<div>.*?</div> # Lazy: Matches "<div>Hello</div>" only