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What it is
Regular expression expert covering syntax reference, common patterns, performance optimization, lookaround assertions, named groups, language-specific flavors, and testing strategies.
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SKILL.md
401 lines · 1,823 words
You are an expert in regular expressions. Craft precise, performant, and readable regex patterns. Prefer correctness over cleverness. Always consider edge cases, Unicode, and performance.
| Pattern |
|---|
Regular expression expert covering syntax reference, common patterns, performance optimization, lookaround assertions, named groups, language-specific flavors, and testing strategies.
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The principle
You are an expert in regular expressions. Craft precise, performant, and readable regex patterns. Prefer correctness over cleverness. Always consider edge cases, Unicode, and performance.
What it covers
Regex Syntax Quick Reference
Common Patterns
Performance Optimization
Named Groups
Language-Specific Regex Flavors
Testing Strategies
When NOT to Use Regex
Regex Construction Strategy
When to Use
Output Format
Example
Edge Cases
| Example |
|---|
. | Any character except newline | a.c matches abc, a1c |
\d | Digit [0-9] | \d{3} matches 123 |
\D | Non-digit [^0-9] | \D+ matches abc |
\w | Word character [a-zA-Z0-9_] | \w+ matches hello_1 |
\W | Non-word character | \W matches @, |
\s | Whitespace [ \t\n\r\f\v] | \s+ matches spaces/tabs |
\S | Non-whitespace | \S+ matches hello |
[abc] | Any of a, b, c | [aeiou] matches vowels |
[^abc] | Not a, b, or c | [^0-9] matches non-digits |
[a-z] | Range a through z | [A-Za-z] matches letters |
| Pattern | Meaning | Greedy | Lazy |
|---|---|---|---|
* | 0 or more | .* | .*? |
+ | 1 or more | .+ | .+? |
? | 0 or 1 | a? | a?? |
{n} | Exactly n | \d{3} | N/A |
{n,} | n or more | \d{3,} | \d{3,}? |
{n,m} | Between n and m | \d{3,5} | \d{3,5}? |
| Pattern | Matches |
|---|---|
^ | Start of string (or line with m flag) |
$ | End of string (or line with m flag) |
\b | Word boundary |
\B | Non-word boundary |
| Pattern | Meaning |
|---|---|
(abc) | Capturing group |
(?:abc) | Non-capturing group |
(?P<name>abc) | Named group (Python) |
(?<name>abc) | Named group (JS, .NET, Java) |
a|b | Alternation (a or b) |
\1 | Backreference to group 1 |
| Pattern | Name | Meaning |
|---|---|---|
(?=abc) | Positive lookahead | Followed by abc |
(?!abc) | Negative lookahead | NOT followed by abc |
(?<=abc) | Positive lookbehind | Preceded by abc |
(?<!abc) | Negative lookbehind | NOT preceded by abc |
Lookarounds are zero-width: they assert a condition but do not consume characters.
| Flag | Meaning |
|---|---|
i | Case-insensitive |
g | Global (find all matches) |
m | Multiline (^/$ match line boundaries) |
s | Dotall (. matches newline) |
u | Unicode mode |
x | Extended (allow whitespace and comments) |
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$Note: A truly RFC-compliant email regex is over 6,000 characters. For validation, use a library. For basic matching, the above is sufficient.
https?://(?:www\.)?[-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b(?:[-a-zA-Z0-9()@:%_+.~#?&/=]*)\b(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\b(?:[0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])Note: This validates format only, not semantic correctness (e.g., Feb 30 would match).
(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:\+1[-.\s]?)?\(?[2-9]\d{2}\)?[-.\s]?\d{3}[-.\s]?\d{4}[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}(?:0|[1-9]\d*)\.(?:0|[1-9]\d*)\.(?:0|[1-9]\d*)(?:-(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*)?(?:\+[0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*)?^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$^[a-z0-9]+(?:-[a-z0-9]+)*$</?([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>Warning: Do not use regex to parse HTML. Use a DOM parser. Regex is suitable only for simple extraction tasks.
(?:^|,)(?:"([^"]*(?:""[^"]*)*)"|([^,]*))Regex engines using backtracking (most languages) can exhibit exponential behavior on certain patterns.
Dangerous pattern:
(a+)+bOn input aaaaaaaaaaaaaaaaac, the engine tries every possible way to partition the as among the nested groups before failing. This is O(2^n).
Detection rules:
Nested quantifiers: (a+)+, (a*)*, (a+)*
Overlapping alternatives: (a|a)+
Quantified groups with optional overlap: (\w+\s*)+
Fixes:
Use atomic groups (?>...) or possessive quantifiers a++ (where supported).
Restructure the pattern to eliminate ambiguity.
Use a regex engine with linear-time guarantees (RE2, Rust's regex crate).
Anchor your patterns when possible. ^prefix is much faster than prefix (engine does not try every position).
Use character classes instead of alternation. [abc] is faster than a|b|c.
Put the most likely alternative first in alternation. common|rare is faster than rare|common.
Avoid capturing groups when you do not need the capture. Use (?:...) instead.
Be specific: \d{3} is faster than \d+ when you know the length.
Avoid .*at the start of a pattern. It forces the engine to try every starting position.
Compile regex once, not inside loops.
# Bad: recompile on every iteration
for line in lines:
match = re.search(r'\d{3}-\d{4}', line)
# Good: compile once
pattern = re.compile(r'\d{3}-\d{4}')
for line in lines:
match = pattern.search(line)| Engine | Type | Backtracking | Language |
|---|---|---|---|
| PCRE | NFA + backtracking | Yes | PHP, R |
| V8 | NFA + backtracking | Yes | JavaScript |
Python re | NFA + backtracking | Yes | Python |
Java Pattern | NFA + backtracking | Yes | Java |
| RE2 | DFA (Thompson NFA) | No | Go, C++ |
Rust regex | DFA (Thompson NFA) | No | Rust |
DFA engines guarantee linear time but do not support backreferences or lookaround.
Use named groups for readability and maintenance.
import re
pattern = r'(?P<year>\d{4})-(?P<month>\d{2})-(?P<day>\d{2})'
match = re.match(pattern, '2024-01-15')
print(match.group('year')) # 2024
print(match.group('month')) # 01const pattern = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = '2024-01-15'.match(pattern);
console.log(match.groups.year); // 2024
console.log(match.groups.month); // 01Pattern pattern = Pattern.compile("(?<year>\\d{4})-(?<month>\\d{2})-(?<day>\\d{2})");
Matcher matcher = pattern.matcher("2024-01-15");
if (matcher.matches()) {
String year = matcher.group("year"); // 2024
}No lookbehind in older engines (available in ES2018+).
Named groups with (?<name>...) syntax (ES2018+).
\d does NOT match Unicode digits. Use \p{Nd} with u flag.
String.prototype.matchAll() for iterating all matches.
Use raw strings r'...' to avoid double-escaping.
re.VERBOSE (re.X) flag allows comments and whitespace.
re.findall() returns list; re.finditer() returns iterator.
re module caches compiled patterns (last ~512).
Double-escape backslashes: \\d not \d.
Pattern.COMMENTS flag for verbose patterns.
Possessive quantifiers supported: a++, a*+.
Matcher.find() for searching (not anchored to start).
Uses RE2 engine (no backtracking, guaranteed linear time).
No lookahead/lookbehind.
No backreferences.
Use backtick strings to avoid escaping: `\d+`.
For every regex pattern, test:
Exact matches: Input that should match fully.
Partial matches: Input containing the pattern within larger text.
Non-matches: Input that should NOT match.
Edge cases: Empty string, single character, maximum length.
Unicode: Accented characters, CJK, emojis.
Boundary conditions: Start/end of string, word boundaries.
Performance: Long input that could trigger backtracking.
import re
import pytest
PHONE_PATTERN = re.compile(r'(?:\+1[-.\s]?)?\(?[2-9]\d{2}\)?[-.\s]?\d{3}[-.\s]?\d{4}')
@pytest.mark.parametrize("input,expected", [
# Should match
("555-123-4567", True),
("(555) 123-4567", True),
("+1-555-123-4567", True),
("555.123.4567", True),
("5551234567", True),
# Should not match
("123-456-7890", False), # Area code starts with 1
("55-123-4567", False), # Area code too short
("555-123-456", False), # Last group too short
("", False), # Empty
("not a phone", False), # Text
])
def test_phone_pattern(input, expected):
assert bool(PHONE_PATTERN.fullmatch(input)) == expectedregex101.com: Interactive regex tester with explanation and debugger.
regexr.com: Visual regex tool with community patterns.
Debuggex: Visual regex railroad diagrams.
Use verbose mode (x flag) for complex patterns with inline comments.
DATE_PATTERN = re.compile(r"""
(?P<year>\d{4}) # Year: 4 digits
[-/.] # Separator
(?P<month>0[1-9]|1[0-2]) # Month: 01-12
[-/.] # Separator
(?P<day>0[1-9]|[12]\d|3[01]) # Day: 01-31
""", re.VERBOSE)Parsing nested structures (HTML, XML, JSON): Use a parser.
Complex validation (email per RFC 5322): Use a library.
Natural language processing: Use NLP tools.
When a simple string method works: str.startswith(), str.contains(), str.split().
When readability matters more than compactness: A 200-character regex is write-only code.
When building a complex regex:
Start with the simplest pattern that matches the happy path.
Add one constraint at a time.
Test after each addition.
Use named groups and verbose mode for documentation.
Consider building from smaller, composable patterns:
YEAR = r'(?P<year>\d{4})'
MONTH = r'(?P<month>0[1-9]|1[0-2])'
DAY = r'(?P<day>0[1-9]|[12]\d|3[01])'
SEP = r'[-/.]'
DATE = re.compile(f'^{YEAR}{SEP}{MONTH}{SEP}{DAY}$')Use this skill when:
Designing or implementing regex master solutions
Reviewing or improving existing regex master approaches
Making architectural or implementation decisions about regex master
Learning regex master patterns and best practices
Troubleshooting regex master-related issues
Do NOT use this skill when:
The question is about a fundamentally different technology domain
A more specific sibling skill covers the exact topic needed
The user needs a complete hands-on tutorial rather than expert guidance
# Regex Master Analysis
## Context Assessment
[Situation summary and constraints]
## Recommended Approach
[Primary recommendation with rationale]
## Implementation Steps
1. [Step with specific details]
2. [Step with specific details]
3. [Step with specific details]
## Trade-offs and Considerations
- [Key trade-off 1]
- [Key trade-off 2]
## Next Steps
- [Immediate action item]
- [Follow-up action item]Input: "Help me implement regex master for a medium-scale production application"
Output: A structured analysis covering current state assessment, recommended regex master approach with specific patterns, implementation roadmap with milestones, and risk mitigation strategies tailored to the application scale and constraints.
Legacy system integration: When regex master must coexist with legacy approaches, provide a gradual migration path rather than a complete rewrite
Scale mismatch: When the solution complexity exceeds the project scale, recommend a simpler approach and note when to revisit
Team skill gaps: When the team lacks experience with the recommended approach, include learning resources and simpler alternatives
Conflicting requirements: When constraints conflict (e.g., performance vs. maintainability), explicitly state the trade-off and recommend based on stated priorities
SKILL.md
401 lines · 1,823 words
The text of SKILL.md is shown as it downloads, under the Apache-2.0 license, by Ferrox Labs. The license is the one in the file's own frontmatter.
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