What are Regular Expressions (aka. Regexes)?

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• An idea on how to match some pattern in some text.
• A tool/language that is available in many places.
• Has many different "dialects"
• Has many different modes of processing.
• The grand concept is the same.
• Uses the following symbols:

() [] {} . * + ? ^ $ | - \ \d \s \w \A \Z \1 \2 \3