Topics

One line: One page per job — validating a field, pulling out a date, splitting a line, moving a pattern between two languages — each pulling together the keywords it needs and the chapters that measured them, with a table of its own where the job has an engine-dependent answer.

Keywords cuts the language by syntax; the chapters cut it by question. This shelf cuts it by what you are trying to do. Where a keyword page answers "what does \b mean in Ruby", a topic page answers "how do I validate a name field, and what will go wrong".

Every table here is made the same way as on the Keywords shelf — one program, tools/kwprobe/run.py ↗, eight engines, the matched text or no or - per cell — so a topic page's claims are measured, not remembered. Three of the pages have no table because their subject is a judgement (performance, choosing an engine, testing); those say so and point at the pages where the measuring was done.

Doing a job

Topic	Level	In one line
Validating input	201	^…$ is a line test that looks like a string test; \A…\z, \d versus [0-9], and (?i) on the KELVIN SIGN
Extracting fields	201	Name the groups, loop in the language: the last-iteration rule and the unset group, with the API per language
Search and replace	201	First or all by default, the template language, the empty match, and when to use a function
Splitting text	201	Who returns a captured delimiter, and four answers to splitting on the empty pattern
Escaping and literal text	201	The fourteen metacharacters, what \q does in seven engines, and six escape functions with three outputs
Multiline text and logs	201	(?m), (?s), Ruby's letters, CRLF, \R, and the first and last line of a text
Common patterns and their traps	201	Email, IPv4, date, decimal, quoted string, number — each with the input it wrongly accepts
Readable patterns	201	(?x), #, names and (?(DEFINE)…), and how to build a pattern from parts where the engine has none of them

Staying out of trouble

Topic	Level	In one line
Security	301	The three regex mistakes that are vulnerabilities — $, (?i), unescaped input — in one table, plus ReDoS
Performance	301	Can it explode, how much per character, how often is it compiled — recorded as counts and cliffs, never timings
Unicode text	301	ASCII, code units, or code points? Ten rows that place every engine, and the five rules that survive all of them
Testing patterns	201	The table every pattern deserves, the eight rows that find bugs, and how this library tests itself

Between engines

Topic	Level	In one line
Portability between engines	301	Refused, compiles-and-means-something-else, or same-construct-different-definition: twelve constructs, and the checklist
Choosing an engine	201	Four families, one guarantee, and five questions — with where ABAP sits
Command-line tools	201	BRE, ERE and PCRE behind three flags; leftmost-longest; no lazy quantifiers; and the shell gets the pattern first