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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>PyParsing Regex Inverter</title>
<link rel="stylesheet" href="https://pyscript.net/releases/2025.11.1/core.css">
<script type="module" src="https://pyscript.net/releases/2025.11.1/core.js"></script>
<style>
body { font-family: sans-serif; margin: 2em; }
#regex-input { width: 300px; }
#output { width: 100%; height: 400px; margin-top: 1em; font-family: monospace; white-space: pre; border: 1px solid #ccc; padding: 0.5em; overflow: auto; }
.controls { margin-bottom: 1em; }
table { border-collapse: collapse; width: 100%; }
table th, table td { border: 1px solid #ddd; padding: 8px; text-align: left; }
table th { background-color: #f2f2f2; text-align: center; }
table.quick-ref td:first-child { text-align: center; }
</style>
</head>
<body>
<h1>Regex Inverter</h1>
<h4>by Paul McGuire, January 2026</h4>
<details>
<summary>Description</summary>
<div>
<p>This page allows you to invert a regular expression, generating strings that match it.</p>
<p><strong>Instructions:</strong> Enter a regular expression in the "Regex" field and specify the maximum number of results you want to see (up to 100,000,000). Click "Invert" or press Enter to generate the matching strings.</p>
<p><strong>Constraints:</strong></p>
<ul>
<li>Unbounded repetition operators <code>+</code> and <code>*</code> are <strong>not supported</strong>.</li>
<li>Replace <code>+</code> or <code>*</code> with explicit <code>{1,n}</code> or <code>{min,max}</code> repetition operators (e.g., use an explicit repetition like <code>[A-Z]{1,4}</code> instead of <code>[A-Z]+</code>, or <code>[A-Z]{,4}</code> instead of <code>[A-Z]*</code>).</li>
<li>For brevity, all generated strings in this utility are limited to 7-bit ASCII characters. By default, Python's <code>re</code> methods will match the full Unicode set, so macros like <code>\d</code> could match numeric digits in other language character sets beyond just the ASCII digits '0' through '9'.</li>
</ul>
<p><strong>Note:</strong> Complex regular expressions or those with large repetition counts may take some time to process.</p>
</div>
</details>
<p>
<details>
<summary>Regular Expressions Quick Reference</summary>
<div>
<table class="quick-ref">
<thead>
<tr><th>Construct</th><th>Description</th></tr>
</thead>
<tbody>
<tr><td><code>.</code></td><td>Any character except newline</td></tr>
<tr><td><code>\d</code></td><td>Digit <code>[0-9]</code></td></tr>
<tr><td><code>\w</code></td><td>Word (identifier) character <code>[a-zA-Z0-9_]</code></td></tr>
<tr><td><code>\s</code></td><td>Whitespace character</td></tr>
<tr><td><code>\D</code></td><td>Non-digit</td></tr>
<tr><td><code>\W</code></td><td>Non-word character</td></tr>
<tr><td><code>\S</code></td><td>Non-whitespace character</td></tr>
<tr><td><code>?</code></td><td>0 or 1 repetition</td></tr>
<tr><td><code>{n}</code></td><td>Exactly <i>n</i> repetitions</td></tr>
<tr><td><code>{n,m}</code></td><td>Between <i>n</i> and <i>m</i> repetitions</td></tr>
<tr><td><code>{,m}</code></td><td>0 to <i>m</i> repetitions</td></tr>
<tr><td><code>[...]</code></td><td>Character class (any of these characters)</td></tr>
<tr><td><code>[^...]</code></td><td>Negated character class</td></tr>
<tr><td><code>|</code></td><td>Alternation (OR)</td></tr>
<tr><td><code>(...)</code></td><td>Grouping</td></tr>
<tr><td><code>(?:...)</code></td><td>Grouping (non-capturing)</td></tr>
<tr><td><code>(?P<name>...)</code></td><td>Grouping (named group)</td></tr>
<tr><td colspan="2">Other common regex features not covered in this utility</td></tr>
<tr><td><code>^</code></td><td>Start of a line</td></tr>
<tr><td><code>$</code></td><td>End of a line</td></tr>
<tr><td><code>\A</code></td><td>Start of string</td></tr>
<tr><td><code>\Z</code></td><td>End of string</td></tr>
<tr><td><code>*</code></td><td>0 or more repetitions (use <code>{,m}</code> to limit repetitions)</td></tr>
<tr><td><code>+</code></td><td>1 or more repetitions (use <code>{1,m}</code> to limit repetitions)</td></tr>
<tr><td><code>\b</code></td><td>Word boundary</td></tr>
<tr><td><code>(?=...)</code></td><td>Positive lookahead</td></tr>
<tr><td><code>(?!...)</code></td><td>Negative lookahead</td></tr>
<tr><td><code>(?<=...)</code></td><td>Positive lookbehind</td></tr>
<tr><td><code>(?<!...)</code></td><td>Negative lookbehind</td></tr>
</tbody>
</table>
</div>
</details>
<p>
<details>
<summary>Examples</summary>
<div>
<p>Here are some example regular expressions to try:</p>
<table>
<thead>
<tr><th>Description</th><th>Regex</th></tr>
</thead>
<tr>
<td>Match one uppercase letter followed by three digits</td>
<td><code>[A-Z]-\d{3}</code></td>
</tr>
<tr>
<td>Time of day (HH:MM:SS)</td>
<td><code>(2[0-3]|[01][0-9]):([0-5][0-9]):([0-5][0-9])</code></td>
</tr>
<tr>
<td>8-bit binary numbers</td>
<td><code>[01]{8}</code></td>
</tr>
<tr>
<td>Integer from 0 to 99</td>
<td><code>[1-9]?\d</code></td>
</tr>
<tr>
<td>Integer from 0 to 255</td>
<td><code>25[0-5]|2[0-4]\d|1\d{2}|[1-9]\d|\d</code></td>
</tr>
<tr>
<td>Roman Numerals to 50</td>
<td><code>(X{,3}|XL)(I{,3}|IV|VI{,3}|IX)|L</code></td>
</tr>
<tr>
<td>Chemical Symbol</td>
<td><code>A[cglmrstu]|B[aehikr]?|C[adeflmorsu]?|D[bsy]|E[rsu]|F[emr]?|G[ade]|H[efgos]?|I[nr]?|Kr?|L[airu]|M[dgnot]|N[abdeiop]?|Os?|P[abdmortu]?|R[abefghnu]|S[bcegimnr]?|T[abcehilm]|U(u[bhopqst])?|V|W|Xe|Yb?|Z[nr]</code></td>
</tr>
<tr>
<td>IPv4 addresses in 192.168.0.0/16</td>
<td><code>192\.168(\.((25[0-5]|2[0-4]\d|1\d{2}|[1-9]\d|\d))){2}</code></td>
</tr>
<tr>
<td>MAC address</td>
<td><code>[0-9A-Fa-f]{2}([:-][0-9A-Fa-f]{2}){5}</code></td>
</tr>
<tr>
<td>UUID</td>
<td><code>[0-9A-F]{8}(-[0-9A-F]{4}){3}-[0-9A-F]{12}</code></td>
</tr>
<tr>
<td>Original US Area codes (leading and trailing digit 2-9, middle digit 1 or 0)</td>
<td><code>[2-9][10][2-9]</code></td>
</tr>
</table>
</div>
</details>
<p>Enter a regular expression to see its matching strings.</p>
<div class="controls">
<label for="regex-input">Regex:</label>
<input type="text" id="regex-input" placeholder="e.g. [A-Z]-\d{3}" value="">
<label for="max-results">Max results:</label>
<input type="number" id="max-results" value="200" min="1" style="width: 60px;">
<button id="invert-btn" py-click="do_invert">Invert</button>
<button id="cancel-btn" py-click="cancel_invert" style="display: none;">Cancel</button>
</div>
<div id="status"></div>
<textarea id="output" readonly></textarea>
<p>GitHub repo for this page <a href="https://github.com/ptmcg/regex_inverter.git" target="_blank">regex-inverter</a></p>
<p>Powered by <a href="https://pyscript.net" target="_blank">PyScript</a> and <a href="https://pypi.org/project/pyparsing/" target="_blank">pyparsing</a></p>
<py-config>
packages = ["pyparsing"]
[[fetch]]
files = ["inv_regex.py"]
</py-config>
<script type="py">
from pyscript import document
from inv_regex import invert, count
import pyparsing
import itertools
import asyncio
is_cancelled = False
def cancel_invert(event):
global is_cancelled
is_cancelled = True
async def do_invert(event):
global is_cancelled
is_cancelled = False
regex = document.querySelector("#regex-input").value.strip()
if not regex:
return
try:
max_results = int(document.querySelector("#max-results").value)
except ValueError:
max_results = 200
if max_results > 100_000_000:
max_results = 100_000_000
document.querySelector("#max-results").value = 100_000_000
output_area = document.querySelector("#output")
status_div = document.querySelector("#status")
cancel_btn = document.querySelector("#cancel-btn")
output_area.value = ""
status_div.innerText = "Processing..."
# Use a small delay to allow status to update in the UI
await asyncio.sleep(0.1)
try:
# Get up to max_results items using an iterator
invert_iter = invert(regex)
num_shown = 0
while num_shown < max_results:
results = list(itertools.islice(invert_iter, min(max_results - num_shown, 100_000)))
num_shown += len(results)
output_area.value += "\n".join(results) + "\n"
if len(results) < 100_000:
break
if num_shown < max_results:
cancel_btn.style.display = "inline"
await asyncio.sleep(0)
if is_cancelled:
status_div.innerText += " (cancelled)"
break
if not is_cancelled:
# Count the remaining items in the iterator
remaining_count = 0
for i, _ in enumerate(invert_iter, 1):
remaining_count = i
if i % 100_000 == 0:
status_div.innerText = f"Counting matches... {num_shown + i:,} found so far"
await asyncio.sleep(0)
if is_cancelled:
status_div.innerText += " (cancelled)"
break
total_count = num_shown + remaining_count
if not is_cancelled:
status_div.innerText = f"Total matching strings: {total_count:,}"
if total_count > max_results:
status_div.innerText += f" (showing first {max_results:,})"
except pyparsing.ParseBaseException as pe:
status_div.innerText = "Error"
output_area.value = f"Parse Error: {pe.msg}\n{pe.explain(depth=0)}"
except Exception as e:
status_div.innerText = "Error"
output_area.value = f"Error: {str(e)}"
finally:
cancel_btn.style.display = "none"
await asyncio.sleep(0)
# Add event listener for Enter key in input box
def on_keypress(event):
if event.key == "Enter":
asyncio.create_task(do_invert(None))
document.querySelector("#regex-input").onkeypress = on_keypress
document.querySelector("#max-results").onkeypress = on_keypress
</script>
</body>
</html>
|
