elasticsearch/docs/reference/scripting-languages/painless/painless-regexes.md

mapped_pages	products
https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-regexes.html	id painless

Regexes [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.

Pattern p = /[aeiou]/

::::{warning} A poorly written regular expression can significantly slow performance. If possible, avoid using regular expressions, particularly in frequently run scripts. ::::

Pattern flags [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 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