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FSCSS 1.2.4: pattern() that listens when you want it to

#fscss #ai #webdev #regex

Ship intent, not only exact API calls.

FSCSS 1.2.4 hardens semantic pattern(): natural-language lines in a stylesheet can expand into real CSS (or FSCSS) when they score high enough against a registered description — optionally pulling values out of the phrase with @match.

The idea 

_patterns.fscss     →  register descriptions + templates
main.fscss          →  @import patterns, write human phrases
compiler            →  score → inject → @match → CSS
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You are not replacing selectors. You add a listener layer: some lines are prose; if they match a pattern, they become declarations or whole blocks at compile time.

That works cleanest when definitions live in a deferred file or module, and the app stylesheet only imports and speaks.

Install 

npm install fscss@1.2.4
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<script src="https://cdn.jsdelivr.net/npm/fscss@1.2.4/runtime.min.js" defer></script>
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CLI for production:

fscss main.fscss main.css
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Anatomy of a pattern 

pattern(threshold: "plain language description", `
  /* CSS or FSCSS template */
`)
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Piece Role
threshold Minimum similarity 0–1. Omit → default 1 (strict).
description Words used for scoring against call lines.
template What gets injected; backticks allow quotes inside CSS.

Calling: a line that isn’t “hard” CSS structure is treated as a phrase — often inside a rule (property pattern) or alone (block pattern).

pattern(0.5: "Hello World card", `
  border-radius: 12px;
  padding: 24px;
  background: linear-gradient(135deg, #667eea, #764ba2);
  color: #fff;
`)

.card {
  hello world card
}
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Defer definitions: module → main

_patterns.fscss (library — no page-specific selectors required):

pattern(0.75: "glass panel elevated", `
  background: color-mix(in srgb, #fff 12%, transparent);
  backdrop-filter: blur(12px);
  border: 1px solid rgba(255,255,255,0.12);
  border-radius: 16px;
`)

pattern(0.6: "rounded primary button with color: white, bg: red", `
  background: @match(background:?\s([#\w\d-_]+)) @match(bg:?\s([#\w\d-_]+));
  color: @match(color:?\s([#\w\d-_]+)) @match(text:?\s([#\w\d-_]+));
  border-radius: 25px;
  padding: 10px 20px;
  font-weight: 700;
  border: 2px solid;
`)
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main.fscss (product code):

@import((*) from "./_patterns.fscss")
/* or registry / URL form your setup uses */

.hero-card {
  glass panel elevated
}

.cta {
  rounded primary button with color: #0BCEAE, background: midnightblue
}
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Why this split helps:

  • Patterns are versioned and shared like design modules.
  • Main files stay readable: intent lines, not giant mixins.
  • Thresholds and @match regexes are tuned once in the library.
  • Teams can contribute patterns without touching every page.

Same idea as @define libraries — different trigger (phrase vs exact name).

Sensitivity: when it “listens”

Similarity is word-oriented and case-insensitive. A phrase must clear the pattern’s threshold and beat other candidates.

Threshold Feel Good for
1 (default) Almost exact Short, generic labels
0.8–0.9 Strict listener Named components
0.5–0.7 Forgiving Longer descriptions
≤ 0.35 Very open Risky; keep wording unique

Sensitive pattern (harder to trigger by accident):

pattern(0.9: "primary cta button solid indigo", `
  background: #4f46e5;
  color: #fff;
  border-radius: 8px;
  padding: 10px 20px;
`)
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Open pattern (more paraphrases work):

pattern(0.5: "Beautiful card", `
  background: #0066ff;
  border-radius: 12px;
`)
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If two descriptions share the same nouns at low thresholds, the wrong template can win. Prefer distinct verbs: create … array vs render … chart.

@match: parameters without a formal signature

@match(regex) runs on the caller’s phrase, not the description. First capturing group wins; else the full match. Nested parentheses in the regex are handled with balanced parsing.

pattern(0.6: "rounded primary button with color: white, bg: red", `
  background: @match(background:?\s([#\w\d-_]+)) @match(bg:?\s([#\w\d-_]+));
  color: @match(color:?\s([#\w\d-_]+)) @match(text:?\s([#\w\d-_]+));
  border-radius: 25px;
  padding: 10px 20px;
`)

.primary {
  rounded primary button with color: #0BCEAE, background: midnightblue
}
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Authors get “arguments” by writing them in prose; the library harvests them. Offer aliases (bg / background, color / text) so one pattern tolerates different wording.

Safety (1.2.4): hostile nested quantifiers and invalid sources are rejected before RegExp runs. Keep captures simple.

Beyond plain CSS: FSCSS in the template

Patterns can emit @arr, module hooks, etc. Example sketch with st-core@v2:

@import((*) from st-core@v2)
@st-root()

pattern(0.35: "create sales array value: 0,", `
@arr @match(create\s([\w-_]+))[
@match(value:?\s([\d\, \s]+))@match(data:?\s([\d\, \s]+))
]
`)

pattern(0.50: "render sales chart with st-core ", `
@st-chart-fill(.fill, @match(render\s([\w-_]+))@match(st-core\s([\w-_]+)))
@st-chart-line(.line, @match(render\s([\w-_]+))@match(st-core\s([\w-_]+)))
.chart {
  @st-chart-points(@match(render\s([\w-_]+))@match(st-core\s([\w-_]+)))
}
`)

create sample array value: 77, 7, 66, 78, 16
render sample chart with st-core

.chart {
  width: 300px;
  height: 200px;
  position: relative;
}
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Flow: human lines → array + chart binds → normal st-core geometry. Tune thresholds so “create” and “render” don’t steal each other’s hits.

Property vs block calls

  • Property: phrase inside .btn { … } → declarations join that rule.
  • Block: phrase alone → @keyframes, top-level FSCSS, large structures. 
pattern(0.7: "animated keyframe for spin", `
@keyframes spin {
  to { transform: rotate(360deg); }
}
`)

an Animated keyframes for spin
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pattern() vs @define

@define pattern()
Trigger Exact @name(args) Fuzzy phrase
Inputs @use(param) @match on free text
Best for Stable module APIs Authoring UX, demos, shared intent libraries

Use both: defines for precise systems (st-core, design tokens); patterns for how people describe common UI.

Workflow that scales

  1. Collect patterns in _patterns.fscss or a published module.
  2. @import into app sheets.
  3. Write natural lines in components/pages.
  4. Compile with CLI for production; use runtime for playgrounds.
  5. Highlight and edit with FSCSS Support in VS Code.

Quick checklist

  • Distinct descriptions; high thresholds for short phrases.
  • Backtick templates; small, safe @match groups.
  • Defer pattern libraries; keep main stylesheets intent-heavy.
  • Prefer 1.2.4+ for matching/@match behavior described here.

Listen on purpose: register what you care about, import it, and speak in the stylesheet. FSCSS turns the hits into CSS — at compile time, under your thresholds.

Source: fscss.devtem.org/pattern

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