Conditional Patterns in Regular Expressions

1. Introduction to Conditional Patterns

Conditional patterns in regular expressions allow you to specify different patterns based on certain conditions. This is particularly useful when you need to match different patterns depending on the presence or absence of specific text or groups.

2. If-Then-Else Patterns

The if-then-else pattern in regular expressions is a conditional construct that allows you to specify different patterns based on a condition. The syntax typically involves an assertion followed by the pattern to match if the condition is true, and another pattern to match if the condition is false.

Example:

Pattern: (?(?=condition)then|else)

Text: "abc123"

Matches: "abc"

Explanation: The pattern checks if the condition (?=condition) is true (e.g., if the text contains "abc"), then it matches "then" (e.g., "abc"), otherwise it matches "else" (e.g., "123").

3. Conditional Matching with Lookaheads

Lookaheads can be used to create conditional patterns by asserting that a certain pattern must or must not precede the main pattern. This allows for more complex conditional matching scenarios.

Example:

Pattern: (?=(a|b))c

Text: "ac"

Matches: "c"

Explanation: The pattern checks if either "a" or "b" precedes "c". If the condition is met, it matches "c".

4. Conditional Matching with Lookbehinds

Lookbehinds can be used similarly to lookaheads to create conditional patterns. They assert that a certain pattern must or must not follow the main pattern.

Example:

Pattern: (?<=a|b)c

Text: "ac"

Matches: "c"

Explanation: The pattern checks if "c" is preceded by either "a" or "b". If the condition is met, it matches "c".

5. Conditional Matching with Groups

Groups can be used to create more complex conditional patterns. By capturing groups, you can create conditions based on the presence or absence of specific groups.

Example:

Pattern: (a)(?(1)b|c)

Text: "ab"

Matches: "ab"

Explanation: The pattern captures "a" in Group 1. If Group 1 is present, it matches "b". If Group 1 is not present, it matches "c".

6. Conditional Matching with Alternation

Alternation can be combined with conditional patterns to create more flexible matching rules. This allows you to specify different patterns based on multiple conditions.

Example:

Pattern: (a|b)(?(1)c|d)

Text: "ac"

Matches: "ac"

Explanation: The pattern captures either "a" or "b" in Group 1. If Group 1 is "a", it matches "c". If Group 1 is "b", it matches "d".

7. Conditional Matching with Quantifiers

Quantifiers can be used in conditional patterns to specify the number of times a pattern must appear before applying the condition.

Example:

Pattern: (a{2})(?(1)b|c)

Text: "aab"

Matches: "aab"

Explanation: The pattern captures "aa" in Group 1. If Group 1 is present, it matches "b". If Group 1 is not present, it matches "c".

8. Practical Use Cases

Conditional patterns are useful in various scenarios, such as validating complex data formats, parsing structured text, and extracting specific information based on context.

Example:

Pattern: (?<=USD|EUR)\d+(?(?=USD)dollars|euros)

Text: "USD100dollars"

Matches: "USD100dollars"

Explanation: The pattern checks if the amount is preceded by "USD" or "EUR". If "USD" is present, it appends "dollars". If "EUR" is present, it appends "euros".