Custom patterns reference

Use specific regular expression syntax to define accurate custom patterns for secret scanning.

Regular expression library for custom patterns

Secret scanning custom patterns are defined using the Hyperscan library and only support Hyperscan regex constructs, which are a subset of PCRE syntax. Hyperscan option modifiers are not supported. For more information on Hyperscan pattern constructs, see Pattern support in the Hyperscan documentation.

Syntax for manually defining custom patterns

The More options section in the UI helps you write regular expressions manually.

Secret format: an expression that describes the format of the secret itself.
Before secret: an expression that describes the characters that come before the secret. By default, this is set to \A|[^0-9A-Za-z] which means that the secret must be at the start of a line or be preceded by a non-alphanumeric character.
After secret: an expression that describes the characters that come after the secret. By default, this is set to \z|[^0-9A-Za-z] which means that the secret must be followed by a new line or a non-alphanumeric character.
Additional match requirements: one or more optional expressions that the secret itself must or must not match.

For simple tokens you will usually only need to specify a secret format. The other fields provide flexibility so that you can specify more complex secrets without creating complex regular expressions.

Example custom pattern

A company has an internal token with five characteristics. They use the different fields to specify how to identify tokens as follows:

Characteristic	Field and regular expression
Length between 5 and 10 characters	Secret format: `[$#%@AA-Za-z0-9]{5,10}`
Does not end in a `.`	After secret: `[^\.]`
Contains numbers and uppercase letters	Additional requirements: secret must match `[A-Z]` and `[0-9]`
Does not include more than one lowercase letter in a row	Additional requirements: secret must not match `[a-z]{2,}`
Contains one of `$%@!`	Additional requirements: secret must match `[$%@!]`

These tokens would match the custom pattern described above:

a9@AAfT!         # Secret string match: a9@AAfT
ee95GG@ZA942@aa  # Secret string match: @ZA942@a
a9@AA!ee9        # Secret string match: a9@AA

These strings would not match the custom pattern described above:

a9@AA.!
a@AAAAA
aa9@AA!ee9
aAAAe9

Limits

Secret scanning supports up to 500 custom patterns for each organization or enterprise account, and up to 100 custom patterns per repository.