Regex Testing & Validation with re-x

Workflow

Follow this cycle when working with regex:

code

Validate → Test → Fix → Check Portability → Apply/Commit

Step 1: Validate syntax

bash

re-x validate 'PATTERN'

Check valid. If false, read error.suggestion for the fix.

Step 2: Test against data

bash

re-x test 'PATTERN' 'sample input'
re-x test 'PATTERN' --file path/to/data.txt
re-x test 'PATTERN' --file big.log --max-matches 10

Check match_count and matches[].text.

Step 3: Check portability

bash

re-x validate 'PATTERN' --target-lang javascript

Read portability to confirm the pattern works in the target language.

Step 4: Preview replacement

bash

re-x replace 'PATTERN' 'REPLACEMENT' 'sample input'

Step 5: Apply to file

bash

re-x apply 'PATTERN' 'REPL' --file path.txt --dry-run
re-x apply 'PATTERN' 'REPL' --file path.txt
re-x apply 'PATTERN' 'REPL' --file path.txt -m  # multiline

Command Reference

Command	Purpose	Key Output Fields
`re-x test PAT INPUT`	Test matching	`matched`, `match_count`, `matches[]`
`re-x test PAT --file F`	Test on file	Same, streaming for large files
`re-x replace PAT REPL INPUT`	Preview replacement	`result`, `replacements_made`
`re-x validate PAT`	Syntax + portability (8 langs)	`valid`, `portability{}`, `engine_required`
`re-x explain PAT`	Structured breakdown	`parts[]` with `token`, `type`, `desc`
`re-x from-examples E1 E2..`	Infer pattern	`inferred[]` with `pattern`, `confidence`
`re-x apply PAT REPL --file F`	Apply to file	`applied`, `replacements_made`, `backup_path`
`re-x benchmark PAT`	Performance check	`catastrophic_backtracking`, `throughput_mb_s`

Output

All commands return JSON by default. Key conventions:

•matched: true/false — whether any match was found
•matches[].start / matches[].end — byte positions (0-indexed)
•matches[].captures[] — capture groups with group, name, text
•Exit code 0 = success (even if no matches). Exit code 1 = error.

Examples

bash

# Validate and test a date pattern
re-x validate '\d{4}-\d{2}-\d{2}'
re-x test '\d{4}-\d{2}-\d{2}' '2024-01-15 meeting'

# Check portability for JavaScript
re-x validate '(?<=\d{3})\w+' --target-lang javascript

# Explain complex regex
re-x explain '^(?:https?://)?(?:www\.)?([^/]+)'

# Infer from examples
re-x from-examples '2024-01-15' '2025-12-31' '2023-06-01'

# Apply replacement with dry-run
re-x apply 'http://' 'https://' --file urls.txt --dry-run
re-x apply 'http://' 'https://' --file urls.txt

# Check for ReDoS
re-x benchmark '(a+)+$' --input 'aaaaaaaaaaab'

Fallback

If re-x is not installed:

bash

grep -cP 'pattern' file.txt                                    # basic match
python3 -c "import re; print(re.findall(r'pattern', 'input'))" # Python

These cannot check portability, detect ReDoS, or return structured output.

Tips

•Use single-quoted raw strings in bash to avoid escaping issues
•--max-matches limits output when testing against large files
•re-x explain helps understand regex in unfamiliar codebases
•Always re-x benchmark before using regex on untrusted user input