MongoDB $regex Operator

What Is $regex?

$regex is a MongoDB query operator that keeps documents whose string field matches a regular expression. Use it in find() or $match for partial text, prefixes, suffixes, and simple patterns.

$regex = field matches this pattern?

{ field: { $regex: "pattern", $options: "i" } }

// mongosh shorthand
{ field: /pattern/i }

^Note  → starts with Note
book$  → ends with book
ash    → contains ash
i      → case insensitive

query filter — not an aggregation extract
(for extract → $regexFind)

How to Use $regex in MongoDB

Write a pattern against a string field, then optionally add flags.

1. Choose the string field to test (for example name).
2. Pass the pattern with { $regex: "..." } (or a /.../ literal).
3. Add $options when you need flags such as "i".
4. Prefer anchored prefixes (^...) when you care about index use.

Pseudocode

for each document:
    text = document.field
    if text is a string AND text matches pattern (with options):
        keep document
    else:
        skip

Cheat Sheet

Keep these facts handy while writing pattern filters.

Live Preview

Test simple patterns against sample product names (same idea as $regex).

pattern = /ash/i

Notebook  → match
Pen Set   → no
Flashcard → match

kept → Notebook, Flashcard

Worked Walkthrough — Names with “ash”

Three products sit in a catalog. We want names that contain ash, ignoring case.

Documents:
  Notebook
  Pen Set
  Flashcard

Filter:
  { name: { $regex: "ash", $options: "i" } }

Notebook  → contains "ash"  ✓
Pen Set   → no "ash"        ✗
Flashcard → contains "ash"  ✓

Result → Notebook, Flashcard

MongoDB Examples

We use three products. First insert them, then filter with $regex, then show a prefix match. Run these in mongosh after inserting the sample data once.

db.products.insertMany([
  { _id: 1, name: "Notebook" },
  { _id: 2, name: "Pen Set" },
  { _id: 3, name: "Flashcard" }
]);

db.products.find()

[
  { _id: 1, name: "Notebook" },
  { _id: 2, name: "Pen Set" },
  { _id: 3, name: "Flashcard" }
]

"ash" + option i

Notebook   ✓  (…ash…)
Pen Set    ✗
Flashcard  ✓  (…ash…)

db.products.find({
  name: { $regex: "ash", $options: "i" }
});

[
  { _id: 1, name: "Notebook" },
  { _id: 3, name: "Flashcard" }
]

db.products.find({
  name: { $regex: "^Note" }
});

// shorthand in mongosh:
// db.products.find({ name: /^Note/ });

{ _id: 1, name: "Notebook" }

Performance Notes

Regex is flexible but easy to make expensive. Anchor prefixes when possible, and consider $text for real search workloads.

Key Takeaways