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Regular Expression

@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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Using regular expressions to search genetic data sciencedirect.com/science/ar…
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Outside a character class, ^ means 'start at the beginning of a line'. Example: /^The/ matches lines that begin with 'The'.
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\d is not always [0-9]. In Unicode-aware engines, \d may match non-ASCII digits. Use an ASCII mode/flag or [0-9] if you mean only 0-9.
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Inside a character class, ^ means 'not'. For example, [^abc] matches any character except a, b, or c.
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Searching Greek and Hebrew with regular expressions johndcook.com/blog/2020/09/2…

Searching Greek & Hebrew with Unicode regular expressions

Some simple regular expression searching with Unicode strings, namely Greek and Hebrew text. Looking for anomalies such as final forms in the middle of words.

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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.
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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…
Image from Regular Expression (@RegexTip)
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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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