Regular Expressions 101

1st Alternative(\b(?'first_word'[a-z](?!([[:alnum:]]*_)(?# the word doesn't contain underscores))(?:[a-z0-9]*)))

Group 1(\b(?'first_word'[a-z](?!([[:alnum:]]*_)(?# the word doesn't contain underscores))(?:[a-z0-9]*)))

\ban ASCII word boundary

Group first_word(?'first_word'[a-z](?!([[:alnum:]]*_)(?# the word doesn't contain underscores))(?:[a-z0-9]*))

Character Class[a-z]

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

Negative Lookahead(?!([[:alnum:]]*_)(?# the word doesn't contain underscores))

Group 3([[:alnum:]]*_)

Character Class[[:alnum:]]*

*matches any number of characters from the set below:

[:alnum:]an alphanumeric character [a-zA-Z0-9]

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

Comment: the word doesn't contain underscores

Non-Capturing Group(?:[a-z0-9]*)

Character Class[a-z0-9]*

*matches any number of characters from the set below:

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

2nd Alternative(\G(?'trailing_word'[A-Z][a-z0-9]*))

Group 4(\G(?'trailing_word'[A-Z][a-z0-9]*))

\Gstart of current search

Group trailing_word(?'trailing_word'[A-Z][a-z0-9]*)

Character Class[A-Z]

A-Zone character from A (65₁₀) to Z (90₁₀) (case sensitive)

Character Class[a-z0-9]*

*matches any number of characters from the set below:

a-zone character from a (97₁₀) to z (122₁₀) (case sensitive)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

(\b(?'first_word'[a-z](?!([[:alnum:]]*_)(?# the word doesn't contain underscores))(?:[a-z0-9]*)))|(\G(?'trailing_word'[A-Z][a-z0-9]*))

New

camelCase
camel
camelCaseCase
camel684Case
ca45melCase
camelCase6523Case
camelCaseCaseCaseCaseCase
camelCCase
PascalCase
// some random commentary with Capitalized letters
camel_Snake_Case

Substitution

\L${first_word}${trailing_word:+_\L${trailing_word}:}