@RegexTip

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

JohnDCook.com
Joined November 2009
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Four tips for learning regular expressions johndcook.com/blog/2008/01/1…

Tips for learning regular expressions

A few realizations that helped me the most when I was learning regular expressions.

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Convert a finite automaton to a regular expression: en.wikipedia.org/wiki/Kleene…

Kleene's algorithm - Wikipedia

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Convert a regular expression to a finite automaton: en.wikipedia.org/wiki/Thomps…

Thompson's construction - Wikipedia

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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. johndcook.com/blog/2026/10/1…
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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? johndcook.com/blog/2019/12/0…

Why can’t grep find negative numbers?

Suppose you're looking for instances of -42 in a file foo.txt. The command grep -42 foo.txt won't work. Instead you'll get a warning message like the following. Usage: grep [OPTION]... PATTERN...

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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.
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Four tips for learning regular expressions johndcook.com/blog/2008/01/1…

Tips for learning regular expressions

A few realizations that helped me the most when I was learning regular expressions.

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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+'.
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+ means one or more occurrences. For example, /ab+c/ matches 'abc' or 'abbc' or 'abbbbbbc'.
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Finding all English words with four or more consecutive vowels: johndcook.com/blog/2019/10/1…

Queueing theory and regular expressions

Queueing theory is the study of waiting in line. That may not sound very interesting, but the subject is full of surprises. For example, when a server is near capacity, adding a second server can cut...

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To include a literal ^ in a character class, don't put it first: /[^^]/ matches any char except '^'.
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