These are my notes on regular expressions. The links below are where I got my info from. Remember that some regex engines will handle regular expressions differently.
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Examples
ipv4 adderss (one line)
(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
good char whitelist for path inputs Linux
^[a-zA-Z0-9\.._/ ]*/$
email address
\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,5}\b
US phone numbers, a 7 or 10 digit number, with extensions allowed, delimiters are spaces, dashes, or periods
^(?:(?:\+?1\s*(?:[.-]\s*)?)?(?:\(\s*([2-9]1[02-9]|[2-9][02-8]1|[2-9][02-8][02-9])\s*\)|([2-9]1[02-9]|[2-9][02-8]1|[2-9][02-8][02-9]))\s*(?:[.-]\s*)?)?([2-9]1[02-9]|[2-9][02-9]1|[2-9][02-9]{2})\s*(?:[.-]\s*)?([0-9]{4})(?:\s*(?:#|x\.?|ext\.?|extension)\s*(\d+))?$
matches any three-character string ending with "at", like "hat" and "cat".
.at
matches "hat" and "cat".
[hc]at
matches all strings matched by .at except "bat".
[^b]at
matches all strings matched by .at other than "hat" and "cat".
[^hc]at
matches "hat" and "cat", but only at the beginning of the string or line.
^[hc]at
matches "hat" and "cat", but only at the end of the string or line.
[hc]at$
matches any single character surrounded by "[" and "]" since the brackets are escaped, for example: "[a]" and "[b]".
\[.\]
match all characters between 2 strings
example = This is not a good sentence.
(?<=This is)(.*)(?=sentence)
I used look behind (?<=) and look ahead (?=) so that "This is" and "sentence" is
not included in the match, but this is up to your use case,
you can also simply write
This is(.*)sentence.
The important thing here is that you activate the "dotall" mode of your regex
engine, so that the . is matching the newline. But how you do this depends on
your regex engine.
The next thing is if you use .* or .*?. The first one is greedy and will match
till the last "sentence" in your string, the second one is lazy and will match
till the next "sentence" in your string.
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Metacharacter Summary
Characters other than . $ ^ { [ ( | ) ] } * + ? \ match themselves.
\ escape char
. any single character
.* anything
? The preceding item is optional and matched at most once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{n} The preceding item is matched exactly n times.
{n,} The preceding item is matched n or more times.
{,m} The preceding item is matched at most m times.
{n,m} The preceding item is matched at least n times, but not more than m times.
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Metacharacter Description
Characters other than . $ ^ { [ ( | ) ] } * + ? \ match themselves.
. Matches any single character (many applications exclude newlines, and exactly
which characters are considered newlines is flavor-, character-encoding-,
and platform-specific, but it is safe to assume that the line feed character
is included). Within POSIX bracket expressions, the dot character matches a
literal dot. For example, a.c matches "abc", etc., but [a.c] matches
only "a", ".", or "c".
[ ] A bracket expression. Matches a single character that is contained within
the brackets. For example, [abc] matches "a", "b", or "c". [a-z] specifies a
range which matches any lowercase letter from "a" to "z". These forms can be
mixed: [abcx-z] matches "a", "b", "c", "x", "y", or "z", as does [a-cx-z].
The - character is treated as a literal character if it is the last or the first
(after the ^) character within the brackets: [abc-], [-abc]. Note that
backslash escapes are not allowed. The ] character can be included in a
bracket expression if it is the first (after the ^) character: []abc].
[^ ] Matches a single character that is not contained within the brackets.
For example, [^abc] matches any character other than "a", "b", or "c".
[^a-z] matches any single character that is not a lowercase letter
from "a" to "z". Likewise, literal characters and ranges can be mixed.
^ Matches the starting position within the string. In line-based tools, it
matches the starting position of any line.
$ Matches the ending position of the string or the position just before a
string-ending newline. In line-based tools, it matches the ending position
of any line.
BRE: \( \)
ERE: ( ) Defines a marked subexpression. The string matched within the
parentheses can be recalled later (see the next entry, \n). A marked
subexpression is also called a block or capturing group.
\n Matches what the nth marked subexpression matched, where n is a digit
from 1 to 9. This construct is theoretically irregular and was not adopted
in the POSIX ERE syntax. Some tools allow referencing more than nine
capturing groups.
* Matches the preceding element zero or more times. For example,
ab*c matches "ac", "abc", "abbbc", etc.
[xyz]* matches "", "x", "y", "z", "zx", "zyx", "xyzzy", and so on.
(ab)* matches "", "ab", "abab", "ababab", and so on.
BRE: \{m,n\}
ERE: {m,n} Matches the preceding element at least m and not more than n times.
For example, a{3,5} matches only "aaa", "aaaa", and "aaaaa". This is not
found in a few older instances of regular expressions.
? Matches the preceding element zero or one time. For example,
ba? matches "b" or "ba".
+ Matches the preceding element one or more times. For example,
ba+ matches "ba", "baa", "baaa", and so on.
| The choice (also known as alternation or set union) operator matches
either the expression before or the expression after the operator. example,
abc|def matches "abc" or "def".
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