Regular Expression @RegexTip
Learn to use regular expressions by following RegexTip. From @JohnDCook. JohnDCook.com Joined November 2009-
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Convert a regular expression to a finite automaton: en.wikipedia.org/wiki/Thompson%…
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Inside a character class, dash means range. For example, [0-9] matches any digit. Outside a character class dash has no special meaning.
Why can't grep find negative numbers? johndcook.com/blog/2019/12/0…
Very important to keep in mind: different rules apply inside versus outside character classes.
Character classes are characters listed inside square brackets. For example, [aeiou] is a character class that matches any vowel.
The characters * and ? have different meanings in regular expressions than they do in shells. Confusingly similar, but different.
What we call 'regular expressions' today includes more general patterns than the original definition from the 1950's.
The escape character \ makes special characters literal. For example, \. matches a period.
Four tips for learning regular expressions johndcook.com/blog/2008/01/1…
.* will match as much as it can, but then backtrack if the next atom can also be matched. Backtracking on some engines is slow.
(Fine print: whether . matches a newline character depends on the implementation and configuration options.)
A dot matches any single character. For example, /r.de/ matches 'ride', 'rode' etc.
+ means one or more occurrences. For example, /ab+c/ matches 'abc' or 'abbc' or 'abbbbbbc'.
Finding all English words with four or more consecutive vowels: johndcook.com/blog/2019/10/1…
? means a character or expression is optional, i.e. zero or one occurrences. Example: /ab?c/ matches 'ac' and 'abc' but not 'abbc'.
* means zero or more occurrences. Ex: /ab*c/ matches 'ac', 'abc', 'abbc', etc.
Languages have minor variations in their regex support. @RegexTip mostly sticks to the common features of Perl, C#, Python, JavaScript, ...
Create a diagram of a regular expression regexper.com
Unicode Regular Expressions Standard: unicode.org/reports/tr18/
'Everybody stand back. I know regular expressions.' xkcd.com/208/
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