Regex Rules

There’s not a lot to know about the rules behind Regex.

Quantifiers

Value	Meaning
n*	0 or more n
n+	1 or more n
n?	0 or 1 n
n{2}	Exactly 2 n
n{2,}	2 or more n
n{,2}	2 or less n
n{2,4}	2, 3 or 4 n

Ranges

Value	Meaning
.	Any character except new line
(A|B)	A or B
(...)	Group
[ABC]	Range (A, B or C)
[^ABC]	Not A, B or C
[A-Z]	Between A and Z
[0-9]	Between 0 and 9
[A-Z0-9]	Between left-sidebarboth A-Z and 0-9
\n	nth group/sub pattern

Anchors and Character Classes

Value	Meaning
^	Start of line
$	End of line
\w	Any word character (includes underscore)
\W	Non-word character
\d	Digit
\D	Non-digit
\s	Whitespace
\S	Not whitespace
\b	Match seperators
\B	Do not match seperators
\0	Nul character
\n	New line

Regex Algebra

Some stuff can be represented in many ways, here are a few.

1. (a + b) + y = a + (b + y)
2. a + b = b + a
3. a + o = o + a = a
4. a + a = a
5. (a . b) . y = a . (b . y)
6. a . ε = ε . a = a
7. a . (b + y) = (a . b) + (a . y)
8. a+ = a . a* = a* . a
9. a* = ε + a+
10. (a + ε)+ = (a + ε)* = a*
11. if X = b + a . X then X = a* . b
12. if X = b + X . a then X = b . a*

RE Module

In class we used the re module from Python. You can find the docs here but I will put a small explination below.

About Python’s regex module, we had to use the following functions.

• match()
• search()
• findall()
• sub()
• split()

match

If zero or more character at the beginning of the string match the pattern it will return the Match. It returns None if the string doesn’t match.

search

Scans through the string and returns the first time the pattern matches. It returns None if it never matches the pattern.

findall

Returns all the non-overlapping matches of a pattern in a string. They are returned as a list of strings in order in which they are found (left-to-right). If more than one group is used the return type will be a list of tuples, each tuples represents one of the groups of the pattern and it’s matches.

sub

Substitues the matches in the string for the characters given to it. It returns the replaced string and if it doesn’t find any match it returns the original string unchanged.

split

Like the name indicates, it splits a string using the regex pattern given to it. It returns a list of strings, those being the new strings generated by the split.

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Next Chapter: Grammars