Learn regular expressions in about 55 minutes

Abstraction: Concise tutorial covering core regex syntax from literals to capture groups

Key points:

• Character classes: . (any char), [abc], [a-z], \d (digit), \w (word char [0-9A-Za-z_]), \s (whitespace), and their negations \D, \W, \S
• Multipliers: {n}, {m,n}, ? (0 or 1), (0+), + (1+); all greedy by default; append ? to make non-greedy (e.g., .?)
• Capture groups with () numbered left-to-right; back-references \1, \2 usable in both replacement expressions and matching (e.g., ([abc])\1 matches aa, bb, cc)
• Anchors: \b word boundary (zero-width), ^ start-of-line, $ end-of-line; some implementations have \A/\z for text boundaries
• Do not use regex to parse HTML/XML (use a parser), validate email addresses (only sending verifies), or validate people's names
• In languages like Java, string escaping adds a layer on top of regex escaping; use Pattern.quote() to escape dynamic input safely

Connections: Regular Expressions · String Processing

Source: http://qntm.org/files/re/re.html