Regex

See https://www.gnu.org/software/sed/manual/html_node/BRE-vs-ERE.html.

See https://www.regular-expressions.info/posix.html.

See http://www.wikiwand.com/en/Comparison_of_regular-expression_engines.

See http://matt.might.net/articles/sculpting-text.

See https://unix.stackexchange.com/questions/119905/why-does-my-regular-expression-work-in-x-but-not-in-y.

BRE Basic Regular Expressions

ERE Extended Regular Expressions

Grep

Perl

POSIX

Sed

Vim

See I Hate Regex.

See JavaScript Regular Expression Visualizer.

See Visual Debugging of Regular Expressions for JavaScript.