---
title: Regexes
description: Regular expression constants are directly supported. To ensure fast performance, this is the only mechanism for creating patterns. Regular expressions...
url: https://www.elastic.co/elastic/docs-builder/docs/2422/reference/scripting-languages/painless/painless-regexes
products:
  - Painless
---

# Regexes
Regular expression constants are directly supported. To ensure fast performance, this is the only mechanism for creating patterns. Regular expressions are always constants and compiled efficiently a single time.
```java
Pattern p = /[aeiou]/
```

<warning>
  A poorly written regular expression can significantly slow performance. If possible, avoid using regular expressions, particularly in frequently run scripts.
</warning>


## Pattern flags

You can define flags on patterns in Painless by adding characters after the trailing `/` like `/foo/i` or `/foo \w #comment/iUx`. Painless exposes all of the flags from Java’s [ Pattern class](https://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.md) using these characters:

| Character | Java Constant            | Example                                     |
|-----------|--------------------------|---------------------------------------------|
| `c`       | CANON_EQ                 | `'å' ==~ /å/c` (open in hex editor to see) |
| `i`       | CASE_INSENSITIVE         | `'A' ==~ /a/i`                              |
| `l`       | LITERAL                  | `'[a]' ==~ /[a]/l`                          |
| `m`       | MULTILINE                | `'a\nb\nc' =~ /^b$/m`                       |
| `s`       | DOTALL (aka single line) | `'a\nb\nc' =~ /.b./s`                       |
| `U`       | UNICODE_CHARACTER_CLASS  | `'Ɛ' ==~ /\\w/U`                            |
| `u`       | UNICODE_CASE             | `'Ɛ' ==~ /ɛ/iu`                             |
| `x`       | COMMENTS (aka extended)  | `'a' ==~ /a #comment/x`                     |