How do I use start-of-line (^) and end-of-line ($) symbols in different Regex implementations?

The start-of-line (^) and end-of-line ($) symbols are used in regular expressions to match the beginning and end of a line, respectively. While their behavior is consistent across many regex implementations, there can be subtle differences depending on the regex engine and settings.

General Usage:

1. Start-of-line (^):
   • Matches the position at the start of a string or line.
   • Example: ^Hello matches “Hello” only if it appears at the start of the string or line.
2. End-of-line ($):
   • Matches the position at the end of a string or line.
   • Example: world$ matches “world” only if it appears at the end of the string or line.

Behavior in Different Implementations:

1. Standard Behavior (Single-Line Mode)

• Most regex engines (e.g., Java, Python, JavaScript, etc.) treat ^ and $ as matching the start and end of the entire string by default.
• Example:
  • Regex: ^foo$
  • Input: "foo" → Matches the whole string.
  • Input: "bar foo baz" → No match.

2. Multiline Mode (m Flag)

• In multiline mode, ^ and $ match the start and end of each line in the string, not just the start and end of the entire string.
• How to enable multiline mode:
  • JavaScript: Use the m flag: /pattern/m
  • Python: Pass re.MULTILINE to re.compile()
  • Java: Use (?m) inline flag
• Example:
  • Regex: ^foo
  • Input:
    CopyEdit

    foo bar
foo baz

    • Without multiline mode: Matches only the first foo at the start of the string.
    • With multiline mode: Matches both foo occurrences at the start of each line.

3. End-of-line and Trailing Newlines

• Some engines treat $ as matching before a trailing newline (\n) at the end of a string.
• Example:
  • Regex: world$
  • Input: "hello world\n" → Matches “world” before the \n.
• Use \z (or similar) in certain engines for an absolute end-of-string match.

Summary of Regex Engines:

Tips:

• Use \A for start-of-string and \z for end-of-string in engines that support them (e.g., Python, Ruby, Java).
• Always clarify if you need to match entire strings or line-by-line when crafting regex.