character classes for regular expressions code example

Example 1: search class regex

class(?=="(?:(?:.*?\s)?btn(?:\s.*?)?)")

Example 2: grel general expression character classes

* metacharacters
* character escapes \
* anchors \A\Z or ^$
* character classes [][^]
* quantifiers *?+{min,max}
* grouping ()
* substitutions $1 or \1