Shell Variable Interpolation Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching shell variable interpolation, ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# Shell Variable Interpolation
# ReDoS-safe | RegexVault — Dev & Systems > Environment

import re

shell_variable_interpolation_pattern = re.compile(r'\$(?:([A-Za-z_][A-Za-z0-9_]*)(?![A-Za-z0-9_{])|\{([A-Za-z_][A-Za-z0-9_]*)(?::-?[^}]+)?\})')

def validate_shell_variable_interpolation(value: str) -> bool:
    return bool(shell_variable_interpolation_pattern.fullmatch(value))

# Example
print(validate_shell_variable_interpolation("$HOME"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Dev & Systems > Environment category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Groups: 1=bare $VAR name, 2=${VAR} brace name. Shell parameter expansion operators: :- (default), := (assign), :? (error if unset), :+ (alternate value). Numeric positional parameters ($1, $2) are not matched.