imranity · April 2, 2023 18:49 · Apr 2, 2023
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+# python regex 
+
+## chap1 : intro to regex
+
+* regex: Regular expressions are text patterns that define the form a text string should have.
+	- useful for email checking patern
+	- matching word "color" and "colour"
+	- extra specific info like postal code
+LOL: "Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems."
+
+* How regex started (birth of grep)
+Ken Thompson's work didn't end in just writing a paper. He included support for these regular expressions in his version of QED. To search with a regular expression in QED, the following had to be written:
+   g/<regular expression>/p
+In the preceding line of code, g means global search and p means print. If, instead of writing regular expression, we write the short form re, we get g/re/p, and therefore, the beginnings of the venerable UNIX command-line tool grep
+
+?: match single char (file?.xml matches file1.xml and file9.xml but not file99.xml)
+* : match any numder of char
+
+in file?.xml:
+literals -> file and xml
+metacharacters -> ? (or '\*' ) 
+### OUR FIRST REGEX
+/a\w*/ ==> matches any word starting with 'a'
+
+### Escaping Metacharacters
+Metachars can coexist but what if need to use metachar as luterals?
+3 ways to do it:
+* escape the metachar by preceding with a backlash 
+* in python , use "re.escape"
+* Quoting with \Q and \E: (not supported in Python)
+
+There are 12 metachar that should be escaped when needed to use as char:
+ \ backslash
+ ^ Caret
+ $ Dollar Sign
+ . Dot
+ | Pipe Symbol
+ ? Question
+ * Asterik
+ + Plus sign
+ ( 
+ )
+ [
+ {
+
+
+## Character class
+Character classes allow us to define a char that will match if any of defined char on set is present
+
+for example to match "license" and "licene" --> /licen[sc]e/
+we can use range of chars [b-e]  or num [2-9]
+Ranges can be combined : [0-9a-zA-z]
+* Negation of ranges [^0-9] match anything not a number but there has to be a char e.g. /hello[^0-9]/ wont match hello as there no char in its place
+
+### Predefined char class
+
+| Element | Description
+|   .     | matches any char except newline
+|   \d    | matches any decimal , equivalent to [0-9]
+|   \D    | matches any non-digit , eq to [^0-9]
+|   \s    | matches any whitespace class: eq to [ \t\n\r\f\v ]
+|   \S    | matches non-whitespace , eq to [ ^ \t\n\r\f\v ]
+|   \w    | matches any alphanumeric eq to [0-9a-zA-Z_]
+
+[^\/\] -> matches any char thats not a backslash or slash
+
+
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