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What are regular expressions (RegEx) and how can they help me?

Regular Expression can be used in a variety of ways in OneCloud. Here is a high-level table of operators to be aware of.

CharacterWhat does it do?ExampleMatch
^

Matches beginning of line

^abc
abc, abcdef.., abc123
$

Matches end of line

abc$
my:abc, 123abc, theabc
.

Match any character

a.c
abc, asg, a123c
|

OR operator

abc|xyz

abc or xyz

(...)

Capture anything matched

(a)b(c)
Captures 'a' and 'c'
[...]

Matches anything contained in brackets

[abc]
a,b, or c
[a-z]

Matches any characters between 'a' and 'z' but does not include any upper case.

[b-z]
bc, mind, xyz
[0-9]Matches any value between 0 and 3[0-3]*3201
{x}

The exact 'x' amount of times to match

(abc){2}
abcabc
{x,}

Match 'x' amount of times or more

(abc){2,}
abcabc, abcabcabc
*

Greedy match that matches everything in place of the *

ab*c
abc, abbcc, abcdc
+

Matches character before + one or more times

a+c
ac, aac, aaac,
?

Matches the character before the ? zero or one times. Also, used as a non-greedy match

ab?c
ac, abc
/

Escape the character after the backslash or create an escape sequence.

a\sc
a c
(matches the space)


Regular Expressions can be extremely complex, but they are very flexible and powerful and can be used to perform operations and comparisons that cannot be done using exact matches. 

Please see the following examples for basic regular expression usage. For a complete description, please visit www.regular-expressions.info.


^' and '$'

First of all, let's take a look at two special symbols: '^' and '$'. These symbols indicate the start and the end of a string, respectively:

"^The"

matches any string that starts with "The".

"of despair$"

matches a string that ends in with "of despair".

"^abc$"

a string that starts and ends with "abc" - effectively an exact match comparison.

"notice"

a string that has the text "notice" in it.

You can see that if you don't use either of these two characters, you're saying that the pattern may occur anywhere inside the string -- you're not "hooking" it to any of the edges.


'*', '+', and '?'

In addition, the symbols '*', '+', and '?', denote the number of times a character or a sequence of characters may occur. What they mean is: "zero or more", "one or more", and "zero or one." Here are some examples:

"ab*"

matches a string that has an "a" followed by zero or more b's ("ac", "abc", "abbc", etc.)

"ab+"

same, but there's at least one "b" ("abc", "abbc", etc., but not "ac")

"ab?"

there might be a single "b" or not ("ac", "abc" but not "abbc").

"a?b+$"

a possible "a" followed by one or more "b"s at the end of the string:

Matches any string ending with "ab", "abb", "abbb" etc. or "b", "bb" etc. but not "aab", "aabb" etc.


Braces { }

You can also use bounds, which appear inside braces and indicate ranges in the number of occurrences:

"ab{2}"

matches a string that has an a followed by exactly two b's ("abb")

"ab{2,}"

there are at least two b's ("abb", "abbbb", etc.)

"ab{3,5}"

from three to five b's ("abbb", "abbbb", or "abbbbb")

Note that you must always specify the first number of a range (i.e., "{0,2}", not "{,2}"). Also, as you might have noticed, the symbols '*', '+', and '?' have the same effect as using the bounds "{0,}", "{1,}", and "{0,1}", respectively.

Now, to quantify a sequence of characters, put them inside parentheses:

"a(bc)*"

matches a string that has an a followed by zero or more copies of the sequence "bc"

"a(bc){1,5}"

one through five copies of "bc."


'|' OR operator

There's also the '|' symbol, which works as an OR operator:

"hi|hello"

matches a string that has either "hi" or "hello" in it

"(b|cd)ef"

a string that has either "bef" or "cdef"

"(a|b)*c"

a string that has a sequence of alternating "a"s and "b"s ending in a "c"


('.')

A period ('.') stands for any single character:

"a.[0-9]"

matches a string that has an a followed by one character and a digit

"^.{3}$"

a string with exactly 3 characters


Bracket expressions

specify which characters are allowed in a single position of a string:

"[ab]"

matches a string that has either an a or a b (that's the same as "a|b")

"[a-d]"

a string that has lowercase letters 'a' through 'd' (that's equal to "a|b|c|d" and even "[abcd]")

"^[a-zA-Z]"

a string that starts with a letter

"[0-9]%"

a string that has a single-digit before a percent sign

",[a-zA-Z0- 9]$"

a string that ends in a comma followed by an alphanumeric character


You can also list which characters you DON'T want -- just use a '^' as the first symbol in a bracket expression (i.e., "%[^a- zA-Z]%" matches a string with a character that is not a letter between two percent signs).

In order to be taken literally, you must escape the characters "^.[$()|*+?{\" with a backslash ('\'), as they have special meaning. On top of that, you must escape the backslash character itself. 

Lastly, please note that bracket expressions are an exception to that rule and inside of them, all special characters, including the backslash ('\'), lose their special powers (i.e., "[*\+?{}.]" matches exactly any of the characters inside the brackets). If there is a need to include a literal ']' in the list, make it the first character (following a possible '^'). To include the literal '-', make it the first or last character, or the second endpoint of a range.

  • Regex is extremely powerful, but could be a little confusing sometimes. 


    Is there any wild card capability available to the exact pattern match?

  • Hi Mike,

    No, exact match expects a literal string.  If you would, I can log an enhancement for wildcarding using asterisks or question marks but outside of that, we have no plans to provide a blend of exact match and regex.  

    Tony

  • Thanks Tony,


    I think it could be helpful in instances where someone might want to search for more that one value that follows a pattern, but is not comfortable with Regular Expressions.

  • Mike,

    I have logged an enhancement request for this.  The product team will need to evaluate this relative to other priorities.  Once I have a product roadmap update that includes this feature, I'll reply back.

    Tony

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