How to Use RegEx for Conditional Substring Search in JavaScript

JavaScript’s RegEx engine offers robust tools for conditional substring searches. While not as feature-rich as some other engines, it is still capable of solving complex tasks with clever pattern construction. This guide covers examples of extracting substrings under specific conditions using JavaScript.

Key Concepts for JavaScript RegEx

1. Condition-Based Search

Extract substrings (e.g., words or numbers) from lines starting with a specific condition, such as the letter a.

Example strings:

1 a apple = fruit + snack  
2 a 42 = number + meaning  
3 pear & apple = fruit + snack

JavaScript RegEx Solution:

1 const regex = /(?<=^a\s).*?\b\w+\b/g;
2 
3 const text = `
4 a apple = fruit + snack
5 a 42 = number + meaning
6 pear & apple = fruit + snack
7 `;
8 
9 const matches = text.match(regex);
10 console.log(matches); // Output: ['apple', '42']

Pattern Breakdown:

• (?<=^a\s) – Positive lookbehind for lines starting with a followed by a space.
• .*?\b\w+\b – Non-greedy search to capture each word or digit after the condition.

2. Searching Within Delimiters

Extract words or numbers enclosed in quotes (" or ') only for lines starting with b.

Example strings:

1 b"banana & orange" = fruit + snack  
2 b"100 = score" + grade

JavaScript RegEx Solution:

1 const regex = /(?<=^b["'])(.*?)(?=["'])/g;
2 
3 const text = `
4 b"banana & orange" = fruit + snack
5 b"100 = score" + grade
6 `;
7 
8 const matches = text.match(regex).flatMap(match => match.match(/\b\w+\b/g));
9 console.log(matches); // Output: ['banana', 'orange', '100', 'score']

Pattern Breakdown:

• (?<=^b["']) – Positive lookbehind for lines starting with b followed by a quote (" or ').
• (.*?) – Non-greedy match for everything inside the quotes.
• match(/\b\w+\b/g) – Extract words or digits from the captured substring.

3. Excluding Nested Patterns

Extract content within backticks () while skipping nested braces (`).

Example string:

1 `{data} processed with {excluded} items and 25 results`

JavaScript RegEx Solution:

1 const regex = /(?<=`)(?:(?!{.*?})[^`])+?(?=`)/g;
2 
3 const text = '`{data} processed with {excluded} items and 25 results`';
4 
5 const matches = text.match(regex).flatMap(match => match.match(/\b\w+\b/g));
6 console.log(matches); // Output: ['processed', 'with', 'items', '25', 'results']

Pattern Breakdown:

• (?<=\) – Positive lookbehind for the backtick character.
• (?:(?!{.*?})[^])+? – Negative lookahead to exclude braces ({}) and stop at the next backtick.
• match(/\b\w+\b/g) – Extract words or digits from the filtered content.

Limitations and Optimizations

1. Limited Lookbehind Support:

For environments without lookbehind, you can rework the pattern by capturing the leading condition explicitly:

1 const regex = /^a["'](.*?)(?=["'])/g;

2. Prevent Backtracking:

Optimize patterns with non-capturing groups to reduce performance overhead caused by excessive backtracking.

3. Hybrid Logic:

Combine RegEx with JavaScript functions for complex scenarios where native features like \K are unavailable.

Conclusion

JavaScript RegEx allows for precise substring searches when combined with creative pattern design. From handling delimiters to ignoring nested structures, these examples demonstrate how to solve practical problems efficiently.

Experiment with these patterns in your codebase, and share your insights!