Regular Expression

Learn to use regular expressions by following RegexTip. From @JohnDCook.

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Four tips for learning regular expressions

https://t.co/6Z3EWrMPnO

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Use explicit whitespace: /\s+/ matches one or more whitespace characters. /[ \t]+/ restricts it to spaces and tabs.

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Inside a character class, period and question mark are not special. For example, [.?] matches a period or a question mark.

[Regular Expression @RegexTip]

Convert a finite automaton to a regular expression: https://t.co/ZbOKLdMquT

[Regular Expression @RegexTip]

Convert a regular expression to a finite automaton: https://t.co/Jmhbm2Gr1T

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Finding words that can be spelled with chemical symbols using a regex.

The regex can either run in linear time or exponential time, depending on the regex engine.

https://t.co/B7ylmKQzY8

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Regex testing tip: include both examples that should match and examples that should not match.

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Why can't grep find negative numbers? https://t.co/IakwWkDP76

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For ASCII-only case-insensitive matching, use an ASCII/locale mode if available because Unicode casefolding can be surprising.

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Very important to keep in mind: different rules apply inside versus outside character classes.

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\B is the negation of a word boundary. For example, /\Bcat\B/ matches 'scathing' (cat inside) but not 'cat' as a standalone word.

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The characters * and ? have different meanings in regular expressions than they do in shells. Confusingly similar, but different.

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Word boundary \b is a position between \w and \W, not a character. /\bcat\b/ matches 'cat' but not 'concatenate'.

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What we call 'regular expressions' today includes more general patterns than the original definition from the 1950's.

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The escape character \ makes special characters literal. For example, \. matches a period.

[Regular Expression @RegexTip]

Four tips for learning regular expressions https://t.co/6Z3EWrMPnO

[Regular Expression @RegexTip]

Regex engines differ on what \s includes (e.g., non-breaking spaces). If exact whitespace matters, specify it explicitly.

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.* will match as much as it can, but then backtrack if the next atom can also be matched. Backtracking on some engines is slow.

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Greedy quantifiers (*, +, {m,n}) match as much as possible, then backtrack if needed. Lazy quantifiers (*?, +?, {m,n}?) match as little as possible.

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Remember: \\ inside a string literal becomes \ in the regex. In Python, raw strings can be cleaner, e.g r'\d+' is easier ro read than '\\d+'.

[Regular Expression @RegexTip]

+ means one or more occurrences. For example, /ab+c/ matches 'abc' or 'abbc' or 'abbbbbbc'.