A positive lookahead assertion asserts true if the pattern inside the lookahead is matched.
The following is its syntax:
(?=...)
For example, d+(?=##) asserts that there must be a string, ##, immediately after matching one or more digits.
- Preface
- Getting Started with Regular Expressions
- Introduction to regular expressions
- Summary
- Understanding the Core Constructs of Java Regular Expressions
- Understanding the core constructs of regular expressions
- Quantifiers
- Basic quantifiers
- Greedy versus reluctant (lazy) matching using quantifiers
- Possessive quantifiers
- Boundary constructs
- Character classes
- Escaping special regex metacharacters and escaping rules inside the character classes
- Predefined shorthand character classes
- Unicode support in Java regular expressions
- Commonly used Unicode character properties
- Negation of the preceding regex directives
- Unicode scripts support
- Double escaping in a Java String when defining regular expressions
- Embedded regular expression mode modifiers
- The placement of embedded modes in a Java regular expression
- Disabling mode modifiers
- Summary
- Working with Groups, Capturing, and References
- Regular Expression Programming Using Java String and Scanner APIs
- Introduction to the Java String API for regular expressions' evaluation
- Using regular expressions in Java Scanner API
- Summary
- Introduction to Java Regular Expression APIs - Pattern and Matcher Classes
- Exploring Zero-Width Assertions, Lookarounds, and Atomic Groups
- Understanding the Union, Intersection, and Subtraction of Character Classes
- Regular Expression Pitfalls, Optimization, and Performance Improvements
- Common pitfalls and ways to avoid them while writing regular expressions
- Do not forget to escape regex metacharacters outside a character class
- Avoid escaping every non-word character
- Avoid unnecessary capturing groups to reduce memory consumption
- However, don't forget to use the required group around alternation
- Use predefined character classes instead of longer versions
- Use the limiting quantifier instead of repeating a character or pattern multiple times
- Do not use an unescaped hyphen in the middle of a character class
- The mistake of calling matcher.goup() without a prior call to matcher.find(), matcher.matches(), or matcher.lookingAt()
- Do not use regular expressions to parse XML / HTML data
- How to test and benchmark your regular expression performance
- Catastrophic or exponential backtracking
- Optimization and performance enhancement tips
- Use a compiled form of regular expressions
- Use a negated character class instead of the greedy and slow .* or .+
- Avoid unnecessary grouping
- Use lazy quantifiers strategically instead of greedy quantifiers that cause excessive backtracking
- Make use of possessive quantifiers to avoid backtracking
- Extract common repeating substrings out of alternation
- Use atomic group to avoid backtracking and fail fast
- Summary
- Common pitfalls and ways to avoid them while writing regular expressions
Positive lookahead
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